We have investigated the effects of substituting extracellular Na+ by choline or K+ on responses of quin2- and fura-2-loaded human platelets to thrombin and platelet-activating factor (PAF). Na+ substitution by choline did not affect the extent of the rise in [Ca2+]i evoked by either agonist. The response to thrombin, but not PAF, was slightly slowed. High K+ did not activate the cells, but the rises in [Ca2+]i evoked by both agonists were slowed and reduced. Shape change evoked by both agonists was little affected by either substitution. Aggregation evoked by PAF was reduced in high K+ but unaffected in choline. Thrombin-induced aggregation was unaffected by either substitution, even when the rise in [Ca2+]i was markedly reduced. The results suggest that the mechanism which generates Ca2+ fluxes in platelets is not voltage-dependent; but high K+ appears to interfere with the influx mechanism.