Temperature-induced structural changes in the cortical region of the garland cell, which is considered to be active in endocytosis, were investigated in a temperature-sensitive, single gene mutant of Drosophila melanogaster, shibirets1 (shi) and wild-type (Oregon-R). At 19 degrees C, both shi and wild type showed similar structural features: an irregularly extended network of labyrinthine channels, coated pits and vesicles, tubular elements and alpha vacuoles. Tannic acid (TA) impregnation showed that coated pits comprised approximately 20-25% of the total coated profiles at 19 degrees C in both shi and wild-type. When flies were incubated in a horseradish peroxidase (HRP) solution for 5 min, organelles such as coated profiles, tubular elements, and alpha vacuoles were labeled. In wild-type at 30 degrees C, minor changes were observed--mainly a decrease in the distribution of the labyrinthine channels and an increase in HRP uptake. On the other hand, in shi at 30 degrees C, the labyrinthine channels were much elongated and their network became far more complex, indicating the expansion of the surface area of the cell. Also, the coated profiles were increased in number while the number of tubular elements was decreased considerably. The TA method showed that almost all of the coated profiles were coated pits, coated vesicles being almost completely absent at 30 degrees C in shi. Furthermore, HRP uptake activity was considerably decreased at 30 degrees C. These structural changes, as well as the reduced HRP uptake activity, were reversible when the temperature was lowered to 19 degrees C. The observations suggest that in the garland cell of shi the conversion of coated pits to coated vesicles, that is, membrane pinch-off, is blocked at high temperature.