We compared the effects of 50 mM Pi on caffeine-induced Ca²⁺ release (Caff-ICR) in mechanically-skinned fast-twitch (FT) and slow-twitch (ST) skeletal muscle fibers of the rat. The time integral (area) of the Caffeine response was reduced by ~57% (FT) and ~27% (ST) after 30 s exposure to 50 mM Pi in either the presence or absence of creatine phosphate (to buffer ADP). Differences in the SR Ca²⁺ content between FT and ST fibers (~40% vs. 100% SR Ca²⁺ content (pCa 6.7), respectively) did not contribute to the different effects of Pi observed above; under-loading the SR of ST fibers so that the SR Ca²⁺ content approximated that of FT fibers resulted in an even smaller (~21%), but not significant reduction in Caff-ICR by Pi. The observed differences between FT and ST fibers described above could arise from fiber type differences in the ability of the SR to accumulate Ca²⁺-Pi precipitate. To test this, fibers were Ca²⁺ loaded in the presence of 50 mM Pi. In FT fibers the maximum SR Ca²⁺ content (pCa 6.7) was subsequently increased by up to 13 times of that achieved when loading for 2 mins in the absence of Pi. In ST fibers, the SR Ca2+ content was only doubled. These data show that Ca²⁺ release in ST fibers was less affected by Pi than FT fibers and this may be due to a reduced capacity of ST SR to accumulate Ca²⁺-Pi precipitate. This may account in part for the fatigue-resistant nature of ST fibers.