1. Neonatal treatment of rats with vitamin D3 resulted in a change of sexual behavior in adulthood. 2. 2.5 mg vitamin D3 completely inhibited the ejaculation of males without any apparent influence on sexual desire. 250 mg vitamin D3 influenced both the desire and ejaculation. 3. Sexual activity of females was depressed by both doses. 4. The experiments demonstrate that vitamin D3, a steroid in structure, given in the critical period of hormonal imprinting may influence steroid hormone-receptor commanded events for life, in a way similar to the effects exhibited by synthetic steroid hormone analogues and benzpyrene in earlier studies.