Protein utilization in young men under circumstances of one or two periods of work and both adequate and surfeit energy intake was determined by nitrogen balance; protein intake was constant at the FAO/WHO (1973) safe level (0.57 g/kg body-weight). Physical activity affected protein utilization negatively by increasing sweat and faecal N losses, and positively by supporting increased energy intake. Efficiency with which surfeit energy improved N utilization (mg N retained/added kJ) was greater under circumstances of increased activity. Changes in body composition as determined by total body potassium and hydrostatic weighing supported the N retention values.