Vegetative cells of Dictyostelium discoideum were synchronized by a size selection method which gave good synchrony without indication of any respiratory perturbation or abnormal cellular appearance. During synchronous growth, fluctuations in respiratory activity and in total cellular protein content were consistently observed (14 experiments). Control cultures in which the entire exponential population was selected for under identical centrifugation conditions employed to produce the synchronous cultures did not show this pattern; neither did cells subjected to anaerobiosis, cold shock or centrifugation, which were performed to induce perturbation. It appears that extensive turnover of cellular proteins accompanied by respiratory fluctuations occur in the absence of perturbation during vegetative growth of Dictyostelium discoideum. This picture may represent the changes occurring in single cells which would not be evident in time-averaged observations of exponential cultures.