1. Slow thermal reactions occurring in the rhodopsin rods of flash-irradiated frog retinas were investigated spectrophotometrically.2. Five substances were identified as reactants: metarhodopsin II, metarhodopsin III, all-trans-retinal, opsin, and all-trans-retinol.3. Quantitative analysis showed that the transition between these substances are not a series of three consecutive reactions.4. An alternative scheme, compatible with the results, consisted of four reactions and involved two parallel pathways for the decay of metarhodopsin II, viz. conversion into metarhodopsin III, and hydrolysis into retinal and opsin.5. The first-order rate constants for the four reactions were as follows: 1.4 x 10(-2) sec(-1) for the conversion of metarhodopsin II into metarhodopsin III; 7.9 x 10(-3) sec(-1) for the hydrolysis of metarhodopsin II; 1.4 x 10(-3) sec(-1) for the hydrolysis of metarhodopsin III; and 2.6 x 10(-3) sec(-1) for the reduction of retinal into retinol (21 degrees C).6. Two other four-parameter schemes involving an equilibrium between metarhodopsin II and metarhodopsin III were also considered. One was found to be incompatible with the results. The other, though adequate, did not describe the data as well as the model summarized in 4 and 5. It also had the peculiar property of requiring that two apparently independent parameters be equated.