Calcium fluxes were studied in fura-2-labeled rat platelets. Thrombin, ADP and ionomycin induced rapid mobilization of internally stored Ca²⁺, which resulted in only a moderate increase of cytosolic [Ca²⁺]_i. Thrombin and ADP stimulated influx of extracellular Ca²⁺, which was monitored as uptake of ⁴⁵Ca²⁺ and of Mn²⁺. With either agonist, the influx of Ca²⁺ magnified the initial increase of [Ca²⁺]_i. Since responses of rat platelets were dependent on external [Ca²⁺], we conclude that Ca²⁺ influx complements the mobilization of internal stores to reach sufficiently high [Ca²⁺]_i for full activation. A regulatory effect of protein kinase C modulators was observed on both agonist-induced elevation of [Ca²⁺]_i and receptor-mediated Ca²⁺ entry.