Aggregation induced in human platelets by thrombin (TH), collagen (COLL) or the Ca++ ionophore, A23187, was blocked by dibucaine and tetracaine. COLL-induced aggregation was blocked at lower concentrations of anesthetics (0.01-0.5 mM) than TH- or A23187-induced aggregation (0.2-2.0 mM). The rate and magnitude of the release of adenosine diphosphate, Ca++ and serotonin was also decreased by anesthetics. Secretion due to COLL, but not to TH, required extracellular Ca++ and was accompanied by increased uptake of 45Ca which was inhibited by local anesthetics. A23187-induced secretion was partially dependent upon external Ca++ and was accompanied by increased 45Ca uptake. Anesthetics increased 45Ca uptake when added before, but not after, A23187. This effect can be explained by postulating that the anesthetics prevent the release of an internal pool of Ca++, thereby affecting the Ca++ gradient between platelet cytoplasm and extracellular fluid. Platelets degramulated, but not aggregated, by exposure to TH in the presence of ethylene glycol bis(beta--aminoethyl ether)-N, N'-tetraacetic acid and plasmin were isolated by sepharose gel filtration. Such platelets did not aggregate with COLL or TH, but did aggregate with A23187-an effect blocked by local anesthetics. Thus platelet aggregation and secretion are independent Ca++-requiring processes, each of which is inhibitable by local anesthetics, presumable by blocking Ca++ influx or the mobilization of intracellular Ca++ stores.