PURPOSE OF REVIEW: Description of new evidence to support the model for how retinoic acid regulates spermatogonial differentiation, male meiosis and the cycle of the seminiferous epithelium. RECENT FINDINGS: It has been known since the 1920s that vitamin A is essential for spermatogenesis. However, only recently has significant progress been made toward understanding how the active metabolite of vitamin A, retinoic acid, regulates spermatogenesis at multiple different differentiation steps, including the onset of meiosis. Current publications suggest that the initiation and maintenance of the cycle of the seminiferous epithelium is linked to retinoic-acid-driving spermatogonial differentiation and meiotic onset. SUMMARY: Retinoic acid appears to act in a pulsatile manner, periodically driving spermatogonial differentiation and meiotic onset at discrete points along testis tubules, and as a result, is likely to be responsible for generating and maintaining the cycle of the seminiferous epithelium.