Four adult male fallow deer were investigated for 1-4 consecutive years to study the relationships between annual changes in testis volume, sperm quality and antler status. Testicular volume started to increase in July/August, peaked just before the rut, declined until December to 50% of maximum, persisted at this level up to February/March and reached minimal volume after antler casting in late April. There was no apparent age effect on the seasonality of testis size fluctuations. Velvet shedding and antler casting occurred at about 80% and 25%, respectively, of maximal testis volume. Spermatozoa had the same general appearance as those of related ruminants. Viable spermatozoa appeared between August and early May which corresponds almost exactly to the time when fallow deer are in hard antler. From September to March sperm quality would fulfil artificial insemination standards for cattle semen. In June and the first half of July 14 out of 15 ejaculates were devoid of any sperm cells. There were no indications of a secondary seasonal peak in values monitored.

No basic differences in size (mean +/- s.d. for at least 300 spermatozoa), shape and ultrastructure of the spermatozoa of fallow deer were detected (1) in comparison to other artiodactyls, (2) between different fallow bucks, and (3) between different months of the fertile season. The total length of the normal spermatozoon was 67.2 +/- 1.2 microns. The flat, paddle-shaped head was 8.2 +/- 0.3 microns long, 4.4 +/- 0.2 microns for the greatest width, 1.9 +/- 0.2 microns for basal width and, approximately 0.7 microns in thickness. The tail measurements were 13.7 +/- 0.3 microns for the midpiece, 0.5 +/- 0.1 microns for the diameter of the midpiece, 42.6 +/- 0.9 microns for the principal piece, and 2.7 +/- 0.6 microns for the endpiece. Spermatozoa with abnormalities such as cytoplasmic remnants and droplets, bent and coiled tails, as well as microcephalic forms were observed.