The involvement of indole-and catecholamines in the hormonal regulation of sexual receptivity has been investigated in ovariectomized female rats. Drugs that reduce 5-hydroxytryptamine, dopamine, and adrenaline or increase noradrenaline neurotransmission tended to facilitate the occurrence of estrous behavior in estrogen-treated females, and drugs having opposite effects tended to inhibit receptivity induced by estrogen and progesterone. Estrogen decreased noradrenaline turnover in cortex and brain stem; progesterone enhanced this effect in brain stem but prevented it in cortex. Both hormones tended to block noradrenaline uptake in hypothalamus in vitro. In a schedule used to induce receptivity, estrogen accelerated serotonin turnover, an effect prevented by progesterone. Thus a number of monoamines may be involved in the control by hormones of estrous behavior. Furthermore, hormones affect both amine turnover and uptake mechanisms.