The effects of the sulfhydryl group inhibitor thimerosal on serotonin (5-HT) transport activity into rabbit blood platelets were investigated, along with its effects on the intracellular concentration of Ca2+ ([Ca2+]i). 3H-5-HT transport activity into rabbit blood platelets was inhibited by treatment with 10(-5) M thimerosal for 30 min, which did not cause 5-HT release from platelets. The thimerosal-induced inhibition of 5-HT transport was antagonized by dithiotheritol. It was suggested that the thimerosal acts as a sulfhydryl inhibitor and inhibits 5-HT transport activity independently of the 5-HT release reaction in our experiment using rabbit blood platelets. As aspirin did not affect thimerosal-induced 5-HT transport inhibition, it was suggested that the thromboxane A2-generating system does not operate in the effect of thimerosal on 5-HT transport into blood platelets. Furthermore, thimerosal induced a transient elevation of [Ca2+]i, which was followed by a sustained increase. In the absence of extracellular Ca2+, thimerosal caused only a transient increase in [Ca2+]i. It was suggested that the elevation of [Ca2+]i consisted of two phases, e.g. a transient phase induced by Ca2+ mobilization from the intracellular store sites and a sustained phase which might be explained by Ca2+ influx from the extracellular environment. In conclusion, thimerosal inhibited 5-HT transport into blood platelets at a concentration which did not induce 5-HT release, and intracellular Ca2+ mobilization might mediate the inhibitory effect of thimerosal on 5-HT transport.