The requirement for receptor occupancy in thrombin-induced secretion in human platelets has been studied. When increasing concentrations of thrombin were added to gel-filtered platelets containing a constant, high concentration of hirudin, dense granule secretion was initiated at lower thrombin concentrations than those required for alpha-granule secretion and aggregation; acid hydrolase secretion required higher concentrations. A 62-fold excess of hirudin produced abrupt stop of dense granule secretion and alpha-granule secretion when added to non-aggregating (no stirring) platelets shortly after thrombin; it had no affect after these secretory process had reached about 30% of their maximal values. Acid hydrolase secretion was, however, abruptly stopped by hirudin at any stage. When the platelets were allowed to aggregate, the three secretory processes increased their rates and were abruptly stopped by hirudin at any stage. Aggregation (optical) occurred slower than dense granule and alpha-granule secretion, and was reversed by hirudin when added before it had reached 30% of its maximum. It is concluded that alpha-granule secretion, like dense granule secretion, only requires a short receptor occupancy to be completed, in contrast to the requirement for sustained occupancy for acid hydrolase secretion. alpha-Granule secretion might, however, require longer occupancy than dense granule secretion. Aggregation is believed to potentiate secretion through close cell contact and the secretion processes were inhibited by hirudin through hirudin's effect on aggregation.