To study the effect of regeneration on the release of creatine kinase (CK) and prostaglandin E2 from muscle, extensor digitorum longus muscles of mice were injected with 50 microliters of BaCl2 solution in saline (1.2% wt/vol). Injected muscles showed almost complete degeneration at 3 days postinjection but had regenerated to approximately the same fiber cross-sectional area as contralateral control muscles by 12 days postinjection. These muscles released reduced amounts of intracellular CK compared with contralateral control muscles in response to excessive isometric contractile activity or treatment with the calcium ionophore A-23187 (20 microM) in vitro. However, regenerating muscles contained a lower total CK activity than contralateral control muscles, which accounted for the reduced CK efflux after experimental damage. Regenerating muscles released an increased amount of prostaglandin E2 compared with control muscles after both damaging stimuli. We conclude that regenerated muscles show no reduced susceptibility to contraction-induced damage (as assessed by CK release) but show an elevated release of prostaglandin E2 in response to contractile activity or an increase in intracellular calcium.