We have investigated the granule fusion events associated with exocytosis in degranulating peritoneal guinea pig eosinophils by time-resolved patch-clamp capacitance measurements using the phase detector technique. Intracellular stimulation with micromolar calcium and GTP gamma S induces a 2- to 3-fold capacitance increase. The main phase of the capacitance increase occurs after a delay of 2-7 minutes and is composed of well-resolved capacitance steps. The number of steps is very close to the number of crystalloid granules contained in a resting cell and the step size distribution with a peak at 9 fF is in excellent agreement with the granule size distribution determined by electron microscopy. The individual granules thus fuse sequentially with the plasma membrane. The stepwise capacitance increase is frequently preceded by an apparently continuous capacitance increase which consists of steps smaller than 4 fF, indicating exocytosis of small vesicles as distinct from crystalloid-containing granules. In some cases the time course of the opening of individual fusion pores could be recorded, and this revealed metastable conductance states below 300 pS but random fluctuations at higher conductance levels. This behaviour suggests that the small fusion pore might be a protein structure similar to an ion channel, which becomes a continuously variable lipid pore at higher conductances. In some cells a significant capacitance decrease was observed which is apparently continuous, suggesting a process of membrane uptake by endocytosis of small vesicles.