Chemically skinned muscle fibers from rat extensor digitorum longus muscle were used to study the effects of uridine triphosphate (UTP) on Ca²⁺uptake and release by the sarcoplasmic reticulum (SR) and on Ca²⁺-activated tensions. Total replacement (2.5 mM) of adenosine triphosphate (ATP) with UTP (i) increased submaximal Ca²⁺-induced tension (pCa 6.2–5.8) but diminished P_o, the maximum tension elicited by pCa 4.2, by ca. 15%; (ii) markedly reduced Ca²⁺uptake by the SR (evaluated by caffeine-elicited tension); and (iii) induced tension in Ca²⁺-loaded fibers. The UTP-induced tension averaged 55% of P_oand its rates of development and decay were considerably slower than those of caffeine-evoked tension. The UTP-induced tension (i) depended on the Ca²⁺-loading conditions; (ii) was reversibiy blocked by brief (15 s) exposures of Ca²⁺-loaded fibers to 5 mM EGTA or by pretreatment with caffeine; (iii) was abolished by functional disruption of the SR with the nonionic detergent Brij-58; and (iv) persisted after blockade of the SR Ca²⁺release channels with ruthenium red. Exposure of Ca²⁺-loaded fibers to UTP depressed the tension elicited subsequently by caffeine, and enhanced the rate of depletion of caffeine-sensitive Ca²⁺stores during soaking in relaxing solutions containing 5 mM EGTA. The UTP-induced tension is attributed to increased release of Ca²⁺from the SR, via a ruthenium red insensitive pathway(s), combined with reduced Ca²⁺uptake by the SR and increased Ca²⁺affinity of the contractile proteins.Key words: skinned muscle fiber, UTP-induced tension, tension–pCa relationship, sarcoplasmic reticulum, calcium transport.