Preference for an estrous female over a non-estrous female evinced by female rats requires dihydrotestosterone plus estradiol.

The effects were studied of long-term treatment with testosterone metabolites (dihydrotestosterone. DHT, and estradiol, E2, in sc Silastic implants) on preference behavior of ovariectomized female rats for an estrous female over a non-estrous female. For measuring this behavior a residential plus-maze was used which harbored two ovariectomized "stimulus" females on the top of peripheral boxes, one of which was made estrus by injection of estradiol benzoate and progesterone. When both steroids (DHT plus E2) were circulating simultaneously they evoked preference for an estrous female, while neither steroid by itself sufficed. In earlier work with adult male rats castrated on the day of birth, E2 was effective in the absence of DHT. This sex difference, therefore, seems to have arisen before birth. Further, administration of DHT alone caused a profound lack of interest in both "stimulus" females, which cannot be fully explained by the reduced locomotor activity which has been found to be induced by DHT in earlier studies.