Four long-term captive adult rhinoceros iguanas (Cyclura cornuta) on display at the Dallas Zoo (Dallas, TX) were maintained for several years in an outdoor enclosure in the summer and in a smaller indoor enclosure in the winter. While fully exposed to sunlight and given the opportunity to supplement their diet with live vegetation each summer, they were deprived of the benefits of full sun exposure, including ultraviolet B (UVB) exposure, and they received no dietary vitamin D for several months each winter. Maintained with this protocol, the animals seemed healthy and they reproduced. However, increasing awareness of the important role of UVB in vitamin D–mediated calcium metabolism and the potential health risk of UVB and vitamin D deprivation stimulated us to focus more on seasonal changes in vitamin D status and possible subtle changes in health status throughout winter. From September 2008 to October 2010, an 85% decline in the serum 25-hydroxyvitamin D₃ (calcidiol) concentrations was measured over each of two winters, and a half-life of 69 days was estimated for the second winter. A higher vitamin D status (calcidiol) was restored and maintained when the iguanas were outdoors and exposed themselves to 1–2 J of UVB per day, although they may have enhanced their dietary calcium and vitamin D activity by eating cactus fruit, too. Serum calcium and phosphorus concentrations remained higher than published reference values for Cyclura spp. A negative relationship between vitamin D status and voluntary UVB exposure during the summers was discovered, suggesting that the iguanas were regulating, rather than continually maximizing, their vitamin D status at levels lower than those recorded for Cyclura in the tropics. Although the iguanas seemed robust physically, had acceptable calcium and phosphorus blood concentrations, and successfully reproduced throughout the 2 yr monitoring period, we do not recommend extended periods of deprivation of UVB and vitamin D supplementation in winter for rock iguanas until minimally sufficient vitamin D status is more rigorously established.