Plasma concentrations of alpha-melanocyte-stimulating hormone (alpha-MSH) were measured by radioimmunoassay at various times during the oestrous cycle of the rat. Variations in plasma alpha-MSH concentrations occurred throughout the oestrous cycle. Plasma alpha-MSH concentrations were low during the day of dioestrus 1 and rose during the evening reaching peak levels at around 02.00 and 06.00 h on dioestrus 2. Plasma alpha-MSH concentrations then fell and remained low throughout the rest of dioestrus 2. A different pattern was observed during pro-oestrus and oestrus. On those days peak alpha-MSH concentrations occurred during the morning and persisted until the onset of the dark period at around 17.00 h. Plasma alpha-MSH concentrations in ovariectomized rats were found to be increased 24 h after administration of a single injection of oestradiol benzoate and at 12 and 36 h after a single injection of progesterone. It is suggested that these ovarian steroids may influence the rhythm in plasma alpha-MSH concentration that occurs during the oestrous cycle.