Insulin-like growth factor I (IGF-I) is involved in the growth, reproduction, and development of vertebrates. In this study, we report plasma IGF-I concentrations throughout the reproductive cycle of the American alligator (Alligator mississippiensis) and consider these concentrations with respect to several reproductive steroid hormones and ions involved in reptilian reproduction. Mean plasma IGF-I concentrations were significantly different among reproductive stages. Maximal plasma concentrations of IGF-I were observed throughout gravidity. Plasma IGF-I concentration was not correlated with plasma concentrations of estradiol-17 beta, calcium, phosphorus, albumin, total protein, or body size, but was correlated with plasma progesterone concentration. We hypothesize that the elevated concentrations of IGF-I in gravid female alligators are due to the synthesis of this hormone for incorporation into eggs as well as residual IGF-I that could function in the stimulation of uterine secretory products.