Synaptosomal protein synthesis has been found to differ depending on whether synaptosomes are recovered from the sucrose gradients by centrifuging at 20,000 g or at 150,000 g. 2. The fraction obtained at 20,000 g incorporated 60% more amino acid into proteins after 30 min of incubation than the one obtained in the same period of time at 150,000 g. 3. The respiratory capacity of synaptosomes obtained at 20,000 g in the presence or absence of glucose as substrate, is also better preserved than that of those obtained at 150,000 g. 4. It is shown that the yield, enzymic content, and nucleic acid composition is the same in both fractions. 5. These results cannot be explained by the presence of inhibitory factors in the 150,000 g fraction or by the presence of better preserved free mitochondria in the 20,000 g fraction. 6. Our findings are best explained by postulating that the synaptosomal metabolic activity is damaged when synaptosomes are pelleted at too high a speed.
