The depressant effect of active shortening was studied during isometric twitch contraction in intact single muscle fibres of the frog at varied tonicity of the extracellular medium. The shortening effect was calculated as the difference in peak redeveloped force after a small (control) and a larger (test) release step and was expressed in per cent of the isometric tetanic force. The solutions were made hypertonic by addition of sucrose (relative tonicity 1.22T and 1.44T) and hypotonic by reduction of NaCl (relative tonicity 0.81T and 0.62T). The shortening induced depression decreased from 13.0 +/- 1.2% in normal Ringer solution to 7.8 +/- 1.3% after immersion of the fibre in 1.22T solution (mean +/- SE, n = 7). This reduction of the depressant effect by shortening was less than half the size of that obtained in 1.44T solution. An increased force depression by shortening, from 12.7 +/- 1.2% to 17.1 +/- 1.5% (mean +/- SE, n = 7), was obtained when normal Ringer was replaced by 0.81T solution. This enhancement was further augmented in 0.62T solution. Experimental evidence is presented supporting the view that the influence of tonicity on the depressant effect of shortening is not due to tonicity induced changes in fibre width. The effect of varied tonicity on the shortening induced depression appears to be essentially related to alterations in intracellular ionic strength.