The oxidation-reduction potential of cytochrome-c has been studied by Paul (1) and by Rodkey & Ball (2). Whereas previous investigations were confined to a narrow pH range around 7, these new measurements extend over a much wider range, designed to reveal any dissociation constants due to heme-linked groups in the molecule. The results are in fairly good agreement, though differing in detail. Ferricytochrome-c has a dissociation constant of nearly 10-7 [pK = 6.86 according to Paul (1); 7.70 according to Rodkey & Ball (2) J. The Eo' = pH curve has a slope of 0.0 between pH 1.75 and 7.70 (Eo' = +0.254 v.) and of 0.060 between pH 7.70 and 10.0. There is some indication that below pH 1. 75 the slope is 0.120 (2). Light absorption measurements reveal for ferrocytochrome-c a pK value of 9.28 (1). Azide, like cyanide (3), forms a well-defined complex with fer­ ricytochrome-c, though not with the ferrous form (4). The com­ pound is characterized by a shift of the absorption maximum from 5300 A to 5400 A, by the appearance of a faint new band at 5750 A and, again in analogy to the cyanide compound, by the disappear­ rance of the band at 6925 A. The azide complex differs from the cyanide complex by the fact that it is highly dissociated and that its formation is almost instantaneous. Whereas cytochrome-c, like methemoglobin, reacts only with the cyanide or azide ions, cyto­ chrome oxidase combines with the undissociated acids. This con­ clusion is based on an analysis of the inhibition as a function of pH (5). Michel & Scheinberg (6) did not observe any beneficial effects of intravenous injections of cytochrome-c on anoxic or cyanide­ poisoned rats, as claimed by Proger et al. (7). Since Albaum et al. (8) had shown that "after cyanide poisoning 50 per cent of rat brain cytochrome oxidase was still functional, an increased supply of cytochrome-c might conceivably have increased the efficiency of