The mode of action of a single injection of testosterone propionate on LH secretion and release has been studied in the castrated ram. In the first experiment, six castrated rams were injected intramuscularly with 400 mg of testosterone propionate, and the plasma LH levels were assayed by radioimmunoassay: on the first day after treatment, the plasma LH levels decreased significantly (P < 0.001), then remained at a very low level for 5–7 days; initial LH levels were restored by day 9. Testosterone propionate thus blocks the discharge of LH. A second experiment was then performed. Castrated rams in groups of 3 were killed either without treatment (control group D0), or 2, 7 or 9 days after the injection of testosterone propionate (groups D2, D7 and D9). In addition to the plasma LH, hypophyseal LH and hypothalamus LRF (LH releasing factor) were measured: the LRF activity was assessed by the quantity of LH released in vitro from the hypophysis of castrated rats blocked with testosterone propionate. The LH released into the incubation medium and the hypophyseal LH of the ram were measured by the ovarian ascorbic acid method and by radioimmunology. The results of the plasma LH assays were comparable to those in the first experiment: plasma LH was significantly decreased in group D2, increased in group D7 and was restored in group D9. Hypophyseal levels, on the other hand, increased in groups D2 and D7, but were comparable to the controls in group D9, indicating that the injection of testosterone propionate had little or no effect on LH secretion. Finally, the LRF activity was considerably reduced in group D2 (P0), but similar to the control values in group D9. Although secretion appeared normal, the decreased plasma LH levels in group D2 indicated that LRF had not been released; consequently the decrease in LRF observed can be interpreted as being due to an inhibition of its synthesis under the influence of testosterone propionate.