In vitro generation of free radicals by xanthine oxidase acting on xanthine as substrate depressed steady-state calcium uptake by canine cardiac sarcoplasmic reticulum vesicles. The effect of free radicals on the calcium-dependent ATPase activity of the SR vesicles was pH dependent. At pH 7.0, ATPase activity was decreased, but at pH 6.4 it was unchanged. Exposure to free radicals increased the passive permeability of the vesicles to calcium. This increase was inhibited by superoxide dismutase (SOD) at pH 7.0, and SOD plus mannitol at pH 6.4. The increased permeability per se was insufficient to explain the effects of free radicals on ATPase activity, since the calcium ionophore A23187 was unable to mimick these effects. Direct measurement of the number and turnover of the pump units indicated that the number of units was unchanged but turnover was decreased by free radicals at pH 7.0. The overall data suggest at least two mechanisms of free radical damage, one associated with an increase in passive permeability and another associated with an as yet undefined change in some specific steps of the ATPase reaction.