In adult male rats, the 5-HT content of the hippocampus and the amygdala was found to exhibit a diurnal rhythm, with the lowest value at 0400 and the highest at 2000 h. The diurnal curve of plasma corticosterone generally parallels the 5-HT content, with the lowest value at 0800 and the highest at 2000 h. PCPA, 300 mg/kg 72 and 48 h before sacrifice, was found to increase the a.m. and decrease the p.m. plasma corticosterone values, so that the level remained relatively constant throughout the day. At both 0800 and 2000 h, the PCPA produced a 50–63 % reduction in the 5-HT content of the amygdala, hippocampus, and hypothalamus. The results suggest that ‘serotoninergic’ neurons play a role in the diurnal fluctuation in pituitary-adrenal function.