Prolonged exercise followed by a brief period of reduced activity has been shown to result in an overshoot in maximal sarcoplasmic reticulum (SR) Ca2+-ATPase activity [maximal velocity ( Vmax)] in rat locomoter muscles (Ferrington DA, Reijneveld JC, Bär PR, and Bigelow DJ. Biochim Biophys Acta 1279: 203–213, 1996). To investigate the functional significance and underlying mechanisms for the increase in Vmax, we analyzed Ca2+-ATPase activity and Ca2+uptake in SR vesicles from the fast rat gastrocnemius muscles after prolonged running (RUN) and after prolonged running plus 45 min of low-intensity activity (RUN+) or no activity (REC45) and compared them with controls (Con). Although no differences were observed between RUN and Con, both Vmaxand Ca2+uptake were higher ( P < 0.05) by 43 and 63%, respectively, in RUN+ and by 35 and 34%, respectively, in REC45. The increase in Vmaxwas accompanied by increases ( P < 0.05) in the phosphorylated enzyme intermediate measured by [γ-32P]ATP. No differences between groups for each condition were found for the fluorescent probes FITC and ( N-cyclohexyl- N1-dimethylamino-α-naphthyl)carbodiimide, competitive inhibitors of the nucleotide-binding and Ca2+-binding sites on the enzyme, respectively. Similarly, no differences for the Ca2+-ATPase were observed between groups in nitrotyrosine and phosphoserine residues, a measure of nitrosylation and phosphorylation states, respectively. Western blots indicated no changes in relative isoform content of sarcoendoplasmic reticulum (SERCA)1 and SERCA2a. It is concluded that the increase in Vmaxof the Ca2+-ATPase observed in recovery is not the result of changes in enzyme nitroslyation or phosphorylation, changes in ATP and Ca2+-binding affinity, or changes in protein content of the Ca2+-ATPase.