1. Pieces of human retina were dissected from eyes enucleated because of malignant tumours. The isolated retinas were perfused by an ionic medium (36 degrees C) and investigated spectrophotometrically.2. Rhodopsin was identified on the basis of its difference spectrum. The maximum absorbance change on bleaching was about 0.1 (lambda = 500 nm).3. The process of bleaching was quantitatively analysed in terms of four slow reactions, viz. (a) conversion of metarhodopsin II into metarhodopsin III, (b) hydrolysis of metarhodopsin II into retinal and opsin, (c) decay of metarhodopsin III to retinal, and (d) reduction of retinal to retinol.4. First-order rate constants for the reactions in 3 were 9 x 10(-3) sec(-1) (a), 3 x 10(-3) sec(-1) (b), 4 x 10(-3) sec(-1) (c), and 3 x 10(-2) sec(-1) (d).5. With two simplified versions of the model summarized in 3 and 4, the description of experimental data was less accurate.