The potentiation of contractile responses during prolonged incubation of frog sartorius muscles in a conventional Ringer solution and in a calcium-free Ringer solution which contained 400 mM of urea, acetamide or ethylene glycol was observed when stimulation with single electric impulses and with caffeine was carried out. When the muscles were exposed to a hypotonic solution the reduction in the amplitude of caffeine contractures or their complete disappearance were registered, whereas the electrical stimulation of muscles in the same medium exerted an increase in the amplitude of contractile responses. The addition of 400 mM glycerol to the hypotonic solution caused an increase in the amplitude of contractions under both types of stimulation.