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Summary Synthetic luteinizing hormone‐releasing hormone (LRH) was injected intravenously in a dose of 100 μg. to test the capacity of the pituitary to respond to LRH with a release of luteinizing hormone (LH) and follicle‐stimulating hormone (FSH) in 16 healthy regularly menstruating women. A total of 28 LRH tests were performed during different stages of the menstrual cycle. Venous blood was obtained before and at frequent intervals during the first two hours after the LRH injection and assayed for LH, FSH and in five tests also for thyrotrophic (TSH) and growth hormone (GH). No consistent change of TSH or GH was observed after LRH. In all the 28 LRH tests there was a significant increase of LH and in tests also of FSH. The mean LH and FSH increase was 292 and 40 per cent, respectively. The maximum response was usually observed after 30 to 45 minutes. There was a pronounced variation in the response to LRH in the course of the menstrual cycle. During the late follicular phase the average LH response was 4‐ to 6‐fold that obtained in the early and mid‐follicular phase. An increased pituitary responsiveness was also found in the luteal phase. It was suggested that this variation in the response to LRH is due to the changes of the ovarian secretion of oestrogens during the menstrual cycle. In one amenorrhoeic woman exogenous oestrogen was found to enhance the LH response to LRH.

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Long-term treatment with the potent and long-acting stimulatory luteinizing hormone-releasing hormone (LRH) analogue D-Ser(TBU)6-EA10-LRH was given to 4 healthy men to study its effects on pituitary gonadotrophin secretion and gonadal function. Five \g=m\g of the LRH agonist was sel f \ x=req-\ administered sc once daily over 17 weeks. Weekly basal blood samples were obtained for determination of follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin (PRL) and testosterone. The gonadotrophin responses to the LRH analogue were also determined during the treatment period. LRH tests were performed after treatment. Seminal fluid specimens were collected during and after treatment. A reduction of the basal serum gonadotrophin and testosterone levels were observed during the treatment period. The FSH and LH responses to the analogue were also diminished. After discontinuation of treatment the gonadotrophin and testosterone concentrations returned to pre-treatment levels within a week. The PRL levels and the seminal fluid specimens did not show any significant changes during the study period. The results suggest that chronic treatment with D-Ser(TBU)6-EA10-LRH has an inhibitory effect on the pituitary gonadotrophin secretion in healthy men. It seems likely that the reduced testosterone level is secondary to the diminished gonadotrophin secretion.LRH and its agonistic analogues have been found to exert paradoxical antifertility effects in experimental animals (Corbin et al. 1978, review). Labrie et al. (1978) reported dramatic degenerative changes of the rat testis after 4 weeks