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Rats with 5-day ovarian cycles were injected daily with 1 mg bromocriptine. This treatment resulted in a change of cycle length from 5 to 4 days and a rapid increase in ovarian weight. The increase in ovarian weight resulted from the accumulation of large numbers of corpora lutea. Normal numbers of corpora lutea were formed during each cycle but luteal bodies did not disappear subsequently. Luteolysis affected only minor foci of luteal tissue and the majority of luteal tissue remained histologically intact throughout the further period of study. The reduction of cycle length from 5 to 4 days occurred when bromocriptine was administered from the day of ovulation only. If treatment was commenced at a later time during the cycle it was not effective.

Treatment with bromocriptine appeared to affect the concentrations of progesterone in the blood during dioestrus. During treatment the rats showed the pattern characteristic for 4-day cycles: typically, the high concentrations of progesterone on the day after metoestrus remained absent. These data suggest (1) that the latter part of the production of progesterone during dioestrus by 'non-functional corpora lutea' is dependent on prolactin and (2) that prolongation of high progesterone production after metoestrus plays an important role in changing the length of the cycle from 4 to 5 days.

Treatment with bromocriptine did not significantly affect the rate of maturation of follicles destined for the next ovulation. It is possible that follicular maturation is not among the critical variables which determine whether normal ovulatory cycles will last for 4 or 5 days.