Previous investigation of conventional isometric twitches of normothermic cat papillary muscle has shown that hypoxia prolongs relaxation, and this prolongation is actually accentuated during early reoxygenation. Our aim was to identify how hypoxia and reoxygenation affect the coupled processes of activation and inactivation that govern the time course of internally generated contractile tension (Ti). Activation and inactivation are modeled as first-order processes with rate constants ka and ki, respectively, and the overall isometric muscle as an underdamped second-order lag system driven by Ti. The analytical expression (To) for the externally recorded tension is dominated by two exponential terms incorporating ka and ki. Accurate least-squares fits of digitized twitches to To yielded estimates of ka and ki at 1- to 3-min intervals during control oxygenation, hypoxia, and early and late reoxygenation. Results follow. Compared with control, normothermic hypoxia prolonged activation [at 15 min ka decreased 61% from control, 35.5 +/- 6 (SE) s-1, P less than 0.05] and accelerated inactivation (at 15 min, ki increased 69% from control, 6.0 +/- 0.5 s-1, P less than 0.05). In early reoxygenation (1-3 min) activation remained impaired and inactivation returned to control levels (ki decreased 16% from control, NS). In late reoxygenation (15 min) both processes reverted to control. Thus inactivation kinetics can be dissociated from activation kinetics. Impaired relaxation in normothermic hypoxia is due to prolonged activation, whereas inactivation is actually accelerated. The further impairment of relaxation in early reoxygenation is due to rapid return of inactivation to control at a time when activation is still prolonged.