The role which two proteolytic enzymes (cathepsin D, CD and calcium-activated protease, CAP) might play in the early anabolic and subsequent catabolic phases of skeletal muscle protein metabolism was investigated in rats fed normal and protein-deficient (50 g/kg) diets. Enzyme measurements were performed on crude homogenate and subcellular fractions of mixed thigh muscle. In normal pregnancy there was no evidence that the changes in muscle protein mass which occurred were assisted by changes in the activities of CD or CAP. CAP activity was, however, reduced throughout protein-deficient pregnancy. Electron micrographs of gastrocnemius muscle samples taken on day 21 of pregnancy suggested increased lysosome numbers in the protein-deficient animals. However, the specific activity of CD in the muscle microsomal-mitochondrial fraction from these animals showed decreased specific activity. Thus, neither CD nor CAP play any major role in releasing amino acids from maternal skeletal muscle for placental and fetal use during protein deficiency. Changes in CAP activity in early pregnancy may indirectly help to protect the fetus from protein deficiency by allowing maternal protein mass to accumulate early in pregnancy for catabolism and use at a later stage.