Activity changes of thiamine pyrophosphatase (TPPase), acid phosphatase (aP), non-specific esterase (nE), acid beta-galactosidase (a beta Gal), beta-glucuronidase (beta-Gluc), and beta-D-N-acetylglucosaminidase (NAG) in follicles during atresia were investigated in the ovaries of mice, rats, Mongolian gerbils, hamsters, guines pigs, rabbits, cats, and pigs. Changes of hydrolase activity were highly enzyme dependent, species-specific and mostly confined to the granulosa. Decrease of TPPase activity and increase of lysosomal enzyme activities during atresia appeared to be true for all mammals. The start of activity changes in the time course of atresia depended on the occurrence of the enzyme in the growing granulosa. Continuous increase of lysosomal enzyme activity appeared in follicles where these enzymes could also be found in the growing granulosa. In contrast, when lysosomal enzyme activity was low or could not be detected in the growing granulosa, increased enzyme activity could only be observed at a time when degenerative processes have already progressed considerably. This distribution pattern suggests that hydrolytic enzymes in the granulosa cells as well as hydrolases of invading macrophages participate in this degenerative process. In some mammals, enzyme activity changes appeared in the cumulus oophorus for the first time in advanced stages of degeneration. In some mammals enzyme activity changes were dependent on the developmental stage of the follicle. This stage dependency argues for an interrelationship between activation of lysosomal enzymes and androgen metabolism.