The rate of protein synthesis through the cell cycle of Schizosaccharomyces pombe has been determined from the incorporation of pulses of [3H]tryptophan in synchronous cultures prepared by selection in an elutriating rotor. This selection procedure caused minimal perturbations as judged by asynchronous control cultures, which had also been put through the rotor. The rate of synthesis showed a periodic pattern rather than a smooth exponential increase. There was a sharp increase in the rate at an 'acceleration point' at about 0.9 of the cycle. Model-fitting by a novel procedure suggests that the average single cell has an increasing rate of protein synthesis for the first 60% of the cycle and a constant rate for the remaining 40%. The same pattern was shown in less extensive experiments with [3H]leucine and [3H]phenylalanine. It was also shown in a series of size mutants, which indicates that the pattern is not size-related, in contrast to earlier work on the rates of synthesis of messenger RNA. However, one large mutant (cdc 2.M35r20) had a significantly earlier acceleration point. Care was taken to justify the assumption that the rate of incorporation of tryptophan was a valid measure of the rate of protein synthesis. A tryptophan auxotroph was used to eliminate the problem of endogenous supply and the size of the metabolic pool was measured through the cycle. This pool did not show cell-cycle related fluctuations. An operational model of the pools is presented.