The effects of starving, refeeding, and restarving rats for different periods on content of sarcoplasmic and contractile proteins and on activity of the Ca2+-dependent proteinase (CAF) in skeletal muscle was determined. Groups of five to six male rats, 8 to 11 weeks old, were starved up to 8 days, refed up to 6 days, and in two experiments, restarved up to 10 days. CAF activity was assayed in P 0-45 crude CAF fractions prepared so as to remove a protein inhibitor of CAF; the assays were demonstrated to be specific for CAF. Sarcoplasmic protein content of rat skeletal muscle changed little until after 6 days of restarvation when it decreased to 68-89% of control level (P less than 0.05). Contractile protein content decreased to 65% of control level (P less than 0.01) after 8 days of starvation, remained at this level for 4 days of refeeding, then increased to approximately 80% of control level after 6 days of refeeding, remained at this level for 2 days of restarvation, and then decreased to approximately 65% of control level (P less than 0.01) after 6 and 8 days of restarvation. Muscle CAF activity did not change during the first 8 days of starvation but increased to 113% above control level (P less than 0.01) after 6 days refeeding and then decreased to only 29% of control level (P less than 0.01) after 8 days of restarvation. These changes in muscle CAF activity are consistent with the proposed role for CAF in initiating metabolic turnover of contractile proteins, but because actual measurements of myofibrillar protein were not made and because in vivo CAF activity is difficult to assess, they do not prove this role for CAF nor do they exclude participation of other proteinases.