Prairie rattlesnakes from Colorado, USA, were subjected to two thermal treatments during hibernation. The control treatment was carried out in a surrogate den in St. Louis, MO, USA and followed the normal seasonal thermal regime. Experimental manipulations were carried out in a second den in St. Louis. These series of manipulations reversed or modified the thermal gradient normally expected in wild dens. Periodic observations of the snakes within the control and experimental dens provided corroborative data to support the hypothesis that entry to and exit from dens is regulated by a reversing thermal gradient within wild dens. © 1981 Springer-Verlag.