Injection of 1 mg bromocriptine at either 08.00 or 16.00 h on the day of oestrus in rats with 5-day oestrous cycles caused a reduction in the duration of progesterone secretion by the corpus luteum during dioestrous, and a shortening of the ovarian cycle by 1 day. These effects were not present when bromocriptine was injected at 08.00 h on the day of metoestrus. The effect of bromocriptine on progesterone secretion by the corpus luteum was reversed by neutralization of the biological activity of LH at dioestrus by injection of 0.5 ml anti-LH serum at 08.00 h at metoestrus. Injection of the antiserum alone prolonged progesterone secretion by the corpus luteum, but had no effect on the length of dioestrus. These results are interpreted as suggesting (1) that prolactin secretion on the afternoon of oestrus protects the corpus luteum of the rat ovarian cycle against the luteolytic effects of LH secretion during early dioestrus and (2) that prolactin stimulates progesterone secretion in the absence of such a luteolytic action. This response of the corpus luteum of the rat ovarian reproductive cycle to prolactin results in 5-day oestrous cycles.