There is a variety of sex determining mechanisms among vertebrates. Many reptiles possess temperature-dependent sex determination (TSD), in which the incubation temperature of the egg determines the sex of the hatchling. The red-eared slider turtle, Trachemys scripta has often been used as a model system for examining the physiology of TSD. In the current study, the expression of Dmrt1 was examined during TSD in this turtle. Dmrt1 is a putative regulator of sex determination/differentiation and has been identified in a variety of vertebrates, including fishes, amphibians, reptiles, birds, and mammals. Specifically, Dmrt1 has been shown to be up-regulated in a male-specific pattern during embryonic development in many vertebrates. In the current study, the expression patterns of Dmrt1 were examined in the developing adrenal-kidney-gonad complexes of T. scripta during embryonic development. Using a quantitative competitive RT-PCR, Dmrt1 was shown to be up-regulated during the thermosensitive period of sex determination in males. In contrast, levels of Dmrt1 remained low in females throughout the thermosensitive period. These data suggest that the up-regulation of Dmrt1 may play a role in male sex determination/sex differentiation during TSD in T. scripta. Copyright © 2003 S. Karger AG, Basel.