Reptiles may bask for reasons other than thermoregulation. One alternative is to regulate homeostasis of vitamin D(3), a hormone essential to the health of many reptiles. Maintenance of vitamin D(3) homeostasis requires either regulation of exposure to ultraviolet radiation (UV) or ingested vitamin D(3). However, the prey items of most vertebrates tend to be low in vitamin D(3). Thus, basking may be the primary opportunity for vitamin D(3) regulation. We tested the hypothesis that panther chameleons (Furcifer pardalis) behaviorally regulate exposure to UV in natural sunlight with high precision, accuracy, and effectiveness. Panther chameleons with low dietary vitamin D(3) intake significantly increased exposure to UV in natural sunlight compared to those with high dietary vitamin D(3) intake. All lizards fed low dietary vitamin D(3) regulated within optimal UV levels with extreme effectiveness (ability to regulate within optimal UV levels relative to available UV). Chameleons of both dietary treatments regulated UV exposure with great precision, exhibiting little variation among individuals within treatments. Our results add to a growing body of literature that empirically demonstrates the importance of basking for nonthermoregulatory purposes and, more specifically, as an integral mechanism for the regulation of a vital hormone, vitamin D(3).