The dynamics of changes in the amplitude of isometric twitches of isolated EDL and SOL muscles of young rats influenced by low molecular non-electrolytes has been studied. Incubation of EDL in hypertonic glycerol or urea solution (400 mM) leads to a 80 and 60% decrease of twitches, respectively, within 15 minutes. During the following 1--2 hours the twitches restore the initial level or exceed it by 60% (urea). Removal of glycerol or urea causes a stable reduce of contraction up to 5--10% within 1 hour. Reincubation in these non-electrolytes increases twitches 3--4 fold during 1.5--2 hours. Alterations of SOL twitches in urea are the same as of EDL, while glycerol even in concentration of 600 mM produces only small changes. This appears to be determined by a higher permeability of slow muscle fibers to glycerol as compared to EDL. It is suggested that the decrease of twitches and their restore in non-electrolyte solution are determined by the hypertonic effect of the latter while reduction of twitches during non-electrolyte removal is caused by alteration of T-system.