Two isoforms of ryanodine receptors are expressed in skeletal muscles, RyR1 and RyR3. We investigated the relative level of expression of RyRs in developing murine skeletal muscles using [3H]ryanodine binding and immunoprecipitation experiments. In the diaphragm RyR3 accounted for 11% of total RyRs in 5-day-old mice and for 3% of total RyRs in 60-day-old mice. In hindlimb muscles, RyR3 accounted for 3% and 1% of total RyRs in 5-day-old and adult mice, respectively. The activity of RyR1 channels in native microsomal vesicles from murine muscles was found to be as low as 35% of that measured after CHAPS exposure, while no inhibition was observed for RyR3. CHAPS sensitivity of recombinant RyR1 and RyR3 expressed in HEK293 cells was also investigated. The activity of recombinant RyR1 but not RyR3 channels was found to be inhibited in native conditions, suggesting that this property may not be dependent on a muscle environment.