AbstractRecent studies have shown that soluble calcium activated proteases (calpains) in brain degrade proteins associated with the cytoskeleton and vary markedly in activity across regions and as a function of development. It was suggested that the observed differences in calpain activity reflect differences in the turnover rate of structural elements. The present study extends this analysis by measuring the properties and activity of calpain in representatives of the five classes of vertebrates with particular emphasis on the mammals.No evidence for proteolysis was found in soluble fractions of fish brains at neutral pH in the presence or absence of added calcium. A substantial calcium‐independentproteolytic activity was found in amphibian brains—the effects of a variety of protease inhibitors indicated that it is also a neurtral thiol (cysteine) protease. Reptilian brains exhibited both calcium‐independent and calcium‐dependent proteolytic activity. Virtually all proteolytic activity in birds (5 species) and mammals (9 species) measured at neutral pH was calcium‐dependent. The endogenous substrates for the calcium activated proteases were very similar in several species of birds and mammals as were the effects of a variety of protease inhibitors. However, the activity of the enzyme, expressed per mg of soluble protein, was highly and negatively correlated with brain size in the mammals. The allometric expression for this relationship was similar to that found for the density of neurons in cerebral cortex as a function of absolute brain size. These results indicate that soluble proteolytic enzymes in brain are differentially expressed among classes of vertebrates and suggest that the turnover of cytoskeletal elements in birds and mammals differs in important ways from that found in fish and amphibians. The results obtained for mammals raise the possibility of a relationship between brain size and the rate at which structural elements are broken down and replaced in this vertebr