It has been demonstrated that fish rhodopsin is less resistant to thermal treatment and it is more readily bleached as compared to bovine rhodopsin. At 18 degrees C, the stability of both rhodopsins decreased in the following sequence: fragments of the outer segments--digitonin extracts-triton X-100 extracts-cetyltrimethylammonium bromide extracts-At higher temperatures (47-56 degrees), this sequence was the following: fragments of the outer segments-digitonin extracts-cetyltrimethylammonium bromide extracts-triton X-100 extracts. Kinetic and thermodynamic parameters (k, Ea, deltaH, delta S*, delta F*) of rhodopsin bleaching at high temperatures were calculated. It is suggested that at 18 degrees C rhodopsin bleaching includes two steps. Possible causes of idfferent stability of the investigated rhodopsins are discussed.