Abstract
Sixteen mature female rats were injected sub-cutaneously with 10 mg of progesterone daily for 14 days. After 7 days from start of treatment, all animals showed persistent dioestrous vaginal smears. By the 3rd day after withdrawal, most animals showed a pro-oestrous smear. When placed with males at this time, twelve of the sixteen treated females mated as indicated by the presence of spermatozoa in the vaginal smear or by the presence of normally cleaved ova, as compared to only one of five of the oil-injected controls. These results, when compared to the mating percentage of normal colony animals, indicate a significant improvement of mating following withdrawal of progesterone treatment.