Plasma concentrations of cisplatin, total platinum, and total filterable platinum were monitored in 24 patients after either 50 or 100 mg/m2 of cisplatin by rapid intravenous injection. Half the patients at each dose were pretreated with mannitol. Total platinum levels declined in a triphasic fashion with a terminal half-life (t1/2)greater than or equal to 24 hr. Both total filterable platinum and cisplatin levels declined in a monophasic manner and exhibited t1/2 of 0.3 to 0.5 hr. The ratio of cisplatin to total filterable platinum in plasma remained constant (0.6 to 0.8) over the time period (2 hr) during which they could be detected, while the ratio of the plasma levels of cisplatin to total platinum decreased continuously from approximately 0.5 at 5 min to approximately 0.10 at 2 hr. Larger doses of cisplatin resulted in higher plasma levels of all three species monitored, and although the increases appeared somewhat less than proportional to dose, terminal plasma slopes were not dose dependent. Neither mannitol nor dose had an effect on the various species ratios, nor did mannitol appear to affect either plasma levels or terminal plasma decline.