Although there is a growing body of literature concerning the physiology of lizard eggs, this information is largely derived from laboratory experiments, and there have been few measurements of the environmental conditions in natural lizard nests. Unlike the nests of most lizards, the nests of some tropical iguanas are obvious and can be observed and experimentally manipulated. Nests of the Cuban iguana Cyclura nubila were instrumented, and weekly measurements of soil temperature, soil water potential, and the concentrations of O2 and CO2 were recorded. Nest temperatures were similar among nests and over a 24 hr cycle with temperatures around 31°C-32°C. However, there was more variability in soil water potential among the nests, as they dried at different rates after a soaking rain near the beginning of incubation. Although the gaseous environments of the nests were distinctly different from aboveground ambient conditions, the levels of O2 and CO2 were less extreme than in the nests of larger reptiles (sea turtles and crocodiles). The CO2 concentrations were found to be related to nest depth and, to a lesser extent, clutch size.