The present experiment examined the effects of a single neonatal injection of 1 mg or 100 micrograms of testosterone propionate (TP) on the sexual behavior and morphology of the female Mongolian gerbil. Four groups were created: vehicle-treated males (VM), 1-mg TP-treated females (HTP), 100-micrograms TP-treated females (LTP), and vehicle-treated females (VF). In adulthood, tests of sexual behavior were carried out after gonadectomy and appropriate hormone replacement therapy. Results indicated that LTP, HTP, and VM animals were significantly less receptive than VF animals. In addition, VM animals displayed significantly more male sexual activity than HTP, LTP, or VF animals. Immediately after the final test for male sexual behavior, subjects were weighed, anogenital distances recorded, and scent glands measured (length and width). Results indicated that a significant degree of morphological masculinization had occurred in HTP subjects for all measures and for LTP subjects for scent gland width and anogenital distance. These findings suggest that in the gerbil, significant morphological masculinization and behavioral defeminization can both occur in teh absence of significant behavioral masculinization.