This study demonstrates that endocytosis in the oocyte of Drosophila melanogaster is reversibly blocked at the stage of pit formation by the temperature-sensitive, single-gene mutant, shibirts1. Uptake of horeradish peroxidase conjugated with wheat-germ agglutinin was observed to be normal in mutant oocytes at 19 degrees C, but was blocked at 29 degrees C. After 10 min at 29 degrees C, there was a build-up of coated pits along invaginations of the plasma membrane. Also, the endosomal compartment consisting of tubules, bulbs, and small yolk spheres, disappeared. Lowering the temperature to 19 degrees C after 10 min at 29 degrees C released a synchronized wave of endocytosis into a cytoplasm cleared of uptake-related organelles. By observing this synchronized wave after exposure to 19 degrees C for varying durations, we determined that endocytosis proceeds as follows: coated pits/vesicles----tubules----small yolk spheres----mature yolk spheres. The observations suggest that these organelles transform one into another within this sequence.