Studies were conducted into the effects on pituitary gonadotrophic hormones in ovariectomized rats of butorphanol, a synthetic morphine derivative which was claimed to be a potent analgesic with few side-effects, in comparison to effects of the naturally occurring alkaloid morphine. For this purpose, 3 groups of ovariectomized rats were used. Rats of the 1st group were injected butorphanol at 2 dose levels (1 or 2 mg/kg body weight [b.w.]. Those of the 2nd group were injected morphine sulphate (10 or 20 mg/kg b.w.). The 3rd group was injected saline and served as control. Blood samples were collected by orbital sinus punctures, just before treatment and 1 hour post injection. Luteinizing hormone (LH) and follicle-stimulation hormone levels were determined in the sera of rats by radio-immuno-assay. The results revealed that morphine, at the 2 dose levels used, produced more than 90% decrease in serum LH concentration, whereas butorphanol produced more than 70% decrease in serum LH levels. Both morphine and butorphanol, at the 2 doses used, produced more than 76% decrease in serum follicle stimulating hormone concentration. It is concluded that butorphanol, the morphinic derivative, has a depressive effect on the synthesis and/or release of gonadotrophic hormones. This inhibitory effect, however, was nearly as potent as that produced by morphine sulphate.