Rainbow trout provide an excellent model for the molecular analysis of muscle function.  For several years I have studied a developmental shift in muscle kinetics and swimming performance in trout.  At the parr-smolt transformation, red or slow-twitch muscle of trout undergoes a shift in contractions kinetics.  The red muscle of younger trout parr has faster rates of activation and relaxation in isometric contractions and has a faster maximum shortening velocity (Vmax) than that of older trout smolts.  Further, parr swim steadily at higher tailbeat frequencies than smolts.   Molecular biological techniques can be used to examine the molecular basis for this developmental shift in muscle function.  Because of the observed variations in Vmax, an ontogenetic shift in the muscle protein myosin heavy chain (MHC) may be responsible for changes in muscle kinetics.  We examined the MHC protein with SDS-PAGE and through cloning and sequencing MHC mRNAs.  SDS-PAGE revealed a significant shift in the pattern of MHC isoforms.  Parr red muscle samples typically have 2-3 forms of MHC, while smolt samples have 1-2 forms.  Densitometry reveals a statistical difference in the intensity of MHC bands between the two stages, suggesting a developmental reduction in MHC isoforms in trout red muscle.  Three MHC mRNAs were subsequently cloned and sequenced, one each from red, white and ventricular muscle.  Through the use of isoform-specific primer pairs, RT-PCR was used to determine the expression patterns of these three mRNAs in trout red muscle.  Parr red muscle consistently expresses both white and red isoforms, and often the ventricular isoform, of MHC.  As compared to parr, a greater proportion of the smolt red muscle samples expressed only the red muscle isoform.  A shift in MHC expression provides a mechanism for the ontogenetic change in muscle function in rainbow trout.