The accumulation of endogenously synthesized non-yolk proteins, and of exogenously derived yolk, was quantitated during oogenesis and embryogenesis ofDrosophila. Rates of non-yolk protein accumulation were calculated, and were correlated with polysome content at each developmental stage. Three distinct phases of non-yolk protein accumulation were observed: 1) relatively slow accumulation, lasting to stage 9 of oogenesis; 2) very rapid accumulation between stages 10 and 12 of oogenesis, when half of the protein of the mature egg is accumulated in less than 4 h; and 3) no further protein accumulation from stage 12 of oogenesis through at least the gastrula stage of embryogenesis. During phases 1 and 2, rates of non-yolk protein accumulation correlate well with the polysome content of egg chambers. Surprisingly, during the entire phase 3 the content of polysomes remains at high levels, even though no detectable protein accumulation occurs. This finding is in agreement with the low levels of protein synthesis that have been measured during early embryogenesis, and strongly suggests that late in oogenesis the efficiency of translation suddenly drops by about 20-fold. Moreover, our results imply that polysome content cannot always be directly correlated with protein synthetic activity.