A method for in vitro fertilization of Rana sylvatica eggs using spermiation induced by mammalian LH-RF is described. Fertilization success correlated with quantity of sperm produced, which was related to male body weight. Fertilization success declined when spermiation was induced in the same male on successive days. The decline in fertilization ability with successive spermiations was quantified into a single index of reproductive potential. Reproductive potential correlated positively with male body weight. The decline in fertilization accompanying successive induced spermiations was interpreted as a depletion of available sperm, rather than decreased responsiveness to the exogenous hormone. The limited total capacity for sperm production is discussed in the context of the natural reproductive biology of the species.