Protein synthesis is cyclic during pupal diapause in Sarcophaga crassipalpis. These cycles are in phase with infradian MOz cycles, which have a periodicity of about 4 days at 25ยฐC. Mean incorporation of [35S]methionine by diapausing pupae was 5.4% during the 2 days of highest MOz but dropped to 1.7% during the 2 days of low MOz. Diapausing pupae treated with a juvenile hormone analog prior to pupariation had a constant high MOz similar to peak values observed in untreated pupae, and such pupae consistently incorporated [35S]methionine at a high rate (7.7%). [35S]Methionine incorporation by nondiapausing pupae and pharate adults was eightfold higher than the peak rates observed during diapause. Autoradiography of in vivo labeled proteins indicated quantitative and qualitative changes in the synthesis of proteins by diapausing pupae during different phases of the MOz cycle. Brains from diapausing pupae labeled in vitro showed higher incorporation at the peak of the MOz cycle than at the nadir of the cycle, but no such differences were detected for integument, fat body, or fat body supernatant. These differences in tissue response indicate that control of protein synthesis during diapause is not cell autonomous, but is a function of the metabolism of the intact organism.