Observations on the sexual responses of 21 female rats, reared in isolation from the age of 20 days to puberty, on the first occasion of their being put, during the receptive phase of oestrus, with sexually excited males. One female did not become receptive during the hours when tests were given and was eventually dropped from the experiment. The mean age of all females tested was 49.9 [plus or minus] 0.855 days. Nineteen individuals copulated within a few seconds after being put with the males. The cause for 1 failure could not be ascertained, although the probability of a mistaken diagnosis of sexual receptivity was suggested. The immediacy of the copulatory response in these females would seem to justify the conclusion that no environmental influences or factors beyond those necessary to insure normal somatic development are required to bring about sexual maturity as manifested by ability to perform the copulatory act during the receptive phase of oestrus. The age at which copulatory ability was manifested in these isolated females was approximately the same as that of females reared with other animals of the same or the opposite sex.