Thermotropic phase transitions (i.e. phase changes induced by heat) were studied at frog ROS membrane level, with the aid of birefringence (phase difference, T) determination. In both dark and light adapted states, an early phase transition was observed (transition temperature, Tt, from +42 divided by 46 degree C). Apparently this change in the physiological state of the membrane precedes the disorientation of the pigment molecules. Following a period of birefringence stabilization a further decrease in birefringence takes place (from +52 divided by 53 degrees C onwards). During the last part of this phase Rho molecules appear to be completely disoriented, as though in solution. The physical changes underlying the transition phenomena seem related to the unbleached and bleached conditions of the ROS; with rising temperatures, the Tt point does not differ significantly in both conditions.