The effect on work performance of a single oral dose of the cardio-selective beta-adrenoreceptor blocking agent, metoprolol, was compared with an equipotent dose of the non-selective agent, propranolol, in the same subjects. A number of biochemical and physiological variables including heart rate, oxygen consumption, ventilation, lactate, free fatty acid and glucose levels were measured. Following exercise in the presence of both active drugs, subjects complained of excessive leg fatique. For the group there was a significant reduction in the total work performed and the maximum heart rate achieved on both drugs. There was a significant correlation between plasma levels of metoprolol, reduction in total work performed and reduction in maximum heart rate. By contrast, after propranolol, there was a wide variation in work performed at a time when the reduction in maximum heart rate was similar for all subjects. This suggests for propranolol that a reduction in heart rate alone is an inappropriate guide to the impairment of work performance. There was a fail in the circulating level of free fatty acids at the end of exercise in the presence of both drugs and it is possible that this biochemical variable contributed to the decrease in work performance.