Abstract 1. 1. Isolated testes of the seastar Asterias vulgaris near the beginning of the annual spermatogonial proliferative phase were incubated with exogenous polyamines: putrescine, spermidine, and spermine. 2. 2. Testes incubated for 1 or 2 days with exogenous polyamines showed a statistically significant increase in incorporation of tritiated thymidine into DNA. 3. 3. The relative effect of exogenous polyamines on thymidine incorporation was most pronounced in testes with low basal (control) levels of thymidine incorporation. 4. 4. These results suggest that polyamines are involved in a mechanism for initiating and maintaining spermatogonial mitotic divisions in the testes of Asterias vulgaris.