While the role of OT in regulating adult behavior data is thoroughly documented, increasing evidence suggests that neonatal exposure to OT can have long-term effects on behavior and physiology. Based upon the role of OT in regulating adult behavior, I predicted that neonatal OT would affect the expression of adult female sociosexual behavior and sexual maturation. Female Sprague-Dawley rats were treated for the first 7 days of life with intraperitoneal injections of either OT (1 mg/g), an OT antagonist (0.1 mg/g), isotonic saline, or handled only. Parameters measured included age of vaginal opening, age of first estrus, a 10-hr paced sex test during first estrus, and body weight on postnatal days 70, 91 and 136. Treatment with OT significantly delayed the age of vaginal opening and first estrus. OTA significantly reduced mating frequency from an expected rate of 33%. There was no effect on weight at any age.