It has been proposed that estrous behavior in the female rat may be under tonic inhibition by a central serotonergi system. Studies conbining estrogen priming and the pharmacological depletion of serotonin have provided some support for this hypothesis. Some evidence, however, is not consistent with this hypothesis. In the present study estrogen primed ovariectomized-adrenalectomized rats were administered p-chlorophenylalanine and were tested for lordosis behavior 66 and 70 hr later. Lordosis was not facilitated. The animals were then administered progesterone and retested at hour 74. PCPA inhibited progesterone-induced lordosis behavior in a dose dependent manner.