A stereological model, which provides quantitative information on the morphology of the platelet release reaction as isolated from platelet aggregation, was developed for the human platelets separated from blood by Mustard's procedure. Three morphologically defined spaces (granules, surface-connected canalicular system S.C.S., cytoplasm) were used to characterize platelet degranulation (with the variation of the volume density of the granules) and contraction (with the variation of the volume density of both granules and S.C.S.). This model was applied to the evaluation of ultrastructural changes associated with the thrombin-induced release reaction. Degranulation and contraction were associated in the platelets which had been allowed to release for 1,5 and 150 sec. Under conditions of the study, prostaglandins E1 (10(-7) M) inhibited both degranulation and contraction (p less than 0,001). Aspirin (10(-4) M) only inhibited contraction (p less than 0.01) and dibutyryl-AMPc (10(-4)) only inhibited granulation (p less than 0,001).