The effects of in vivo stimulation via the sciatic nerve on Na+, K+ and calcium contents in slow-twitch and fast-twitch muscles were compared. Whereas intermittent stimulation for 24 h at 20 Hz caused only minor changes in soleus (SOL), a considerable loss of K+ (around 24%) and gain of Na+ (around 84%) was observed in extensor digitorum longus (EDL) and tibialis anterior (TA) muscles. These changes could be detected within 0.5 h and a plateau was maintained from 2 to 24 h. Total calcium content increased progressively, reaching values 245 and 382% above the control level in EDL and TA muscle, respectively, after 24 h of 20 Hz stimulation. Whereas the Na+ and K+ content recovered within a few hours, calcium content did not return towards control level until after 48 h of rest. In a pilot study performed with continuous stimulation at 10 Hz, the changes in Na+ and K+ contents in SOL, EDL and TA muscle were comparable to those at 20 Hz. The concentration of the Na(+)-K+ pumps was highest in the fast-twitch EDL and TA muscles and was unaffected by 10 Hz stimulation. It is concluded that a stimulation pattern leading to a rise in intracellular Na+ and a loss of K+ may cause a marked accumulation of calcium. These events seem to be related to insufficient activation of the Na-K+ pump rather than to variations in the total Na(+)-K+ pump capacity.