Summary The results of performing 1720 blood tests for equine cryptorchidism are described. Using the paired sample human chorionic gonadotrophin (hCG) stimulation test and measuring testosterone, 6.7 per cent of tests did not give a clear result. If only the testosterone concentration in the pre‐hCG blood sample was used, this percentage rose to 14 per cent. The paired sample hCG stimulation test was 94.6 per cent accurate. A comparison was made between the paired hCG stimulation test and the measurement of conjugated oestrogen in a single sample. The latter did not give as many doubtfuls but gave false negatives in donkeys of all ages and in horses less than three years old. Measurement of conjugated oestrogen was found to be 96 per cent accurate when donkeys and young horses were excluded. It is, therefore, recommended as a simpler and more reliable test than the paired sample hCG test, but only in the older horse.

Summary Six horses with monorchidism, identified at surgery for cryptorchidectomy, are reported. All six presented with a single scrotal testis. Following surgical removal of one testis, they were either hormonally, anatomically or behaviourally determined to be geldings. Three other horses reported in the literature are reviewed. Of these nine cases of monorchidism, eight were thought to be caused by testicular degeneration and one by testicular agenesis. The vaginal process was present in all of the former and absent in the latter. The left side was involved in five of these eight horses. In seven, the epididymis was absent and, in the remaining two, only the epididymal tail was present. The condition was thought to be congenital in the six horses in this series. A surgical approach to identify accurately monorchid horses is described.

SUMMARY Concentrations of testosterone in the peripheral plasma of normal, cryptorchid and castrated male horses have been measured by a rapid radioimmuno‐assay. Basal concentrations in horses with one or two testicles range from 65 pg to 1600 pg/ml. Intravenous injection of 12000 i.u. of Human Chorionic Gonadotrophin (HCG) into horses possessing testicular tissue stimulated a rise in testosterone concentration which could be detected within 25–35 minutes of the injection. Geldings consistently showed low testosterone concentrations in the plasma (15.3±4.9 pg/ml; 15 animals), and injection of HCG did not stimulate a significant rise in testosterone concentration. Of 29 horses presented as rigs all those which were later found to have been already castrated had plasma testosterone concentrations indistinguishable from those found in geldings (nine animals) and failed to respond to HCG injection (five animals). Evidence is presented that the epididymis and spermatic cord of the horse are not capable of producing significant quantities of testosterone. The possibility is discussed of using paired blood samples, one taken prior to and the other taken after HCG injection, as a test of whether or not a horse has been castrated. RÉSUMÉ Les concentrations de testosterone dans le plasma périphérique des chevaux males normaux, cryptorchides et castrés ont été mesurés par des tests radio actifs. La concentration de base chez les chevaux avec un ou deux testicules varie de 65 pg à 1600 pg/ml. L'injection intraveineuse de 12000 U.I. de gonadotrophine chorionique humaine chez des chevaux possédant un tissu testiculaire entraina. L'augmentation du taux de testostérone détectable entre 25 et 35 minutes après l'injection. Les chevaux hongres montrèrent de basses concentrations plasmatiques de testostérone (15.3±4.9 pg—15 sujets) et l'administration de gonadotrophine ne provoqua aucune élévation significative du taux de testostérone chez eux. Des 29 chevaux réputés cryptorchides tous ceux qui furent identifiés plus tard comme ayant été castrés montrèrent des concentrations plasmatiques de testostérone de même ordre que les hongres, et ne présentérent aucune réponse à l'injection de gonadotrophine. Il semble que ni le lépididyme ni le cordon spermatique du cheval ne soient capables de secrèter des quantités significatives de testostérone. On envisage la possibilité, en comparant deux échantillons de sang, l'un prélevé avant, l'autre prélevé aprés l'injection de gonadotrophine chorionique, d'établir si un cheval a été ou n'a pas été castré. ZUSAMMENFASSUNG Die Testosteronkonzentrationen im peripheren Plasma normaler, kryptorchider und kastrierter männlicher Pferde wurden gemessen mit Hilfe eines schnellen radio‐immuno‐assay. Die Konzentrationen schwanken zwischen 65 bis 1600 pg/ml bei Pferden mit einem oder zwei Hoden. Die intravenöse Injektion von 12'000 IU menschlichen Choriongonadotropins stimulierte einen Anstieg der Testosteronkonzentration bei Pferden mit Hodengewebe nach 25–35 Minuten. Wallachen zeigten immer tiefe Testostero...

SUMMARYThe authors analyse data collected over 23 years from 500 cryptorchid horses. They show that left-and right-sided abdominal cases occur with approximately equal frequency in ponies. Approximately half the right-sided unilateral abdominal cases have the epididymal tail descended while only 20 per cent of the left-sided cases do. These findings are briefly discussed.From their analysis of inguinal cryptorchidism the authors conclude that it is a relatively more complex phenomenon with incidence changing with age as well as breed. Right-sided retention predominates in young ponies, probably being an extreme expression of testicular hypoplasia, but in older ponies and in other types of horse, retention occurs equally on the left and right.