The effects of feeding triiodothyronine on reproductively inhibited prairie deermice (Peromyscus maniculatus bairdii) from laboratory populations.

1. Two reproductively inhibited populations were given 250 ng triiodothyronine (T3) per gram of food for 35 days to elevate the reduced serum thyroid hormone concentration previously demonstrated in reproductively inhibited animals and to determine if the treatment would promote recovery. 2. Per capita food intake was significantly increased but per cent body fat tended to decrease and body weight did not significantly increase during T3 treatment. 3. Mean testis and seminal vesicle weights of T3-treated males were significantly increased and testis histology indicated some increase in spermatogenesis. 4. Mean ovary weights of T3-treated females only tended to increase although the uterine weights were significantly increased and ovarian histology revealed a significant increase in preovulatory follicles. 5. The limited recovery of reproductively inhibited animals may indicate that thyroid activity per se is related to, but not the direct cause of, the reproductive inhibition observed in populations.