The contractile properties of the adductor pollicis and quadriceps muscles have been studied before and 4 and 10 d after major abdominal surgery in 15 patients. Eight patients were considered well nourished and 7 malnourished on the grounds of body mass index, recent weight loss and serum albumin. There were no significant changes in the contractility of the adductor pollicis at 4 and 10 d post-operatively compared to pre-operative values. The only change in the quadriceps was seen in the malnourished patients and was a potentiation of the force:frequency relationship at day 4, affecting force generation at 1 Hz. There was a tendency for the malnourished patients to generate higher forces at low stimulation frequencies and to have slower relaxation rates, but there was no significant difference between the two groups. Pre-operative contractile properties of those patients who developed post-operative complications were not different from those who were free of complications, nor did the post-operative results of the two groups of patients differ. The patients generated higher forces at lower stimulation frequencies than is usually found in young normal subjects. This may be due to age, disease or nutritional factors, but it is not possible to determine the relative contributions of these factors from this study.