Abstract Muscle function tests of the triceps brachii muscle were performed before operation and on the third postoperative day in ten patients undergoing elective cholecystectomy. Electromyograms (EMGs) were recorded by surface electrodes during sustained isotonic and isometric muscle contraction with a constant force of 20 per cent of the preoperative maximal voluntary contraction. Root-mean-square of the EMG was calculated together with the neuromuscular efficiency and measures of the fatiguability. These parameters were compared with changes in the simultaneously measured serum phosphate concentrations. Mean(s.e.m.) neuromuscular efficiency measured after 32–40 s of muscle contraction decreased 14(5) per cent after operation (P < 0·01), whereas the mean fatiguability of the muscle was unchanged. Mean serum phosphate concentration was 0·87(0·06) mmol/l before operation and 0·79(0·06) mmol/l 3 days after the operation (P > 0·05). Two patients developed severe postoperative hypophosphataemia (serum phosphate concentration < 0·50 mmol/l). Postoperative muscle function deterioration was not associated with changes in the serum phosphate level (P > 0·10; r = 0·03). We conclude that patients undergoing cholecystectomy develop postoperative deterioration in skeletal muscle function, which is not associated with serum phosphate concentration.