The role of 5-hydroxytryptamine (5-HT) in the control of sexual receptivity in female rhesus monkeys has been studied in 24 adult females paired with 6 adult males. p-Chlorophenylalanine (PCPA, 75 mg/kg or 100 mg/kg, every fourth day), a selective inhibitor of 5-HT, was found to reverse unreceptivity induced by adrenalectomy in ovariectomised, oestrogen-treated females. PCPA-treated females presented more frequently and initiated more sexual behaviour, or else they refused fewer of the male's attempts to mount. These effects were in turn reversed by 5-hydroxytryptophan (5-HTP, 20 mg/kg every second day), when this was given to PCPA-treated animals. In addition, 5-HTP given alone to ovariectomised oestrogen-treated females reduced their receptivity. Parallel biochemical experiments showed that PCPA in the doses used lowered the levels of 5-HT in the brain as measured by the levels of 5-hydroxyindole-3-acetic acid (5-HIAA) in the CSF, and that these were restored by 5-HTP. Both oestradiol benzoate (15 mug/day for 10 days) and testosterone propionate (250 mug/day or 400 mug/day for 10 days) lowered the turn-over rates of 5-HT in the brain (as measured by the probenecid test) in ovariectomised female monkeys. These effects of oestradiol on turnover were antagonised by progesterone (15 mg/day for 10 days, given with oestradiol). A substance other than an adrenal androgen has thus been found to reverse the effects of adrenalectomy on sexual receptivity in female monkeys. It is therefore possible that androgens regulate receptivity in female monkeys by modifying the activity of 5-HT-containing neural systems.