1. Nitric oxide (NO) donors inhibit platelet function and Ca2+ mobilization evoked by different agonists. This led us to investigate the direct effects of authentic NO on basal cytosolic Ca2+ concentration ([Ca2+]i) and on Ca2+ mobilization induced by thrombin or by two inhibitors of intracellular Ca(2+)-ATPases, thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydroquinone (t-BuBHQ). 2. Cytosolic Ca2+ concentration was evaluated with Fura-2, in the absence of Ca2+ influx. Addition of 5 microM NO increased by 48% the basal cytosolic [Ca2+] of resting human platelets whereas a lower concentration (0.1 microM) did not induce significant modifications. This NO-induced Ca2+ increase was inversely correlated with the basal level of cytosolic [Ca2+]. 3. NO pretreatment for 30 to 120 s decreased by 42 to 57% the transient [Ca2+]i peak evoked by 0.10 u ml-1 thrombin and strongly attenuated the initial rate of [Ca2+]i rise induced by 1 microM thapsigargin or by 20 microM t-BuBHQ. The two components of the thapsigargin response, the Ca2+ release due to inhibition of Ca2+ pumps and the thromboxane A2-dependent self-amplification mechanism, were inhibited by NO. The observation that a subsequent stimulation was still capable of eliciting Ca2+ release suggests the presence of NO-insensitive Ca2+ stores. 4. These findings indicate that nitric oxide can modulate basal cytosolic [Ca2+] in unstimulated human platelets and decrease the Ca2+ mobilization from NO-sensitive internal stores evoked by stimulation of receptors or by Ca(2+)-ATPase inhibitors. This underlines the important role of nitric oxide in the modulation of platelet Ca2+ handling.