The urinary excretion of 3-methylhistidine (3MH), creatinine and total nitrogen was measured during the course of energy balance studies on 20 black Kenyan children. Studies were carried out over 24 h during an acute attack of measles with a second (control) study after recovery, and complete urinary collections were obtained on 14 ill and 18 recovered children. The nutritional status was assessed by anthropometry, and energy intake was determined by duplicate diet analysis. Twelve out of 13 acutely ill and 6 out of 17 recovered children were in negative energy balance. No effect on the rate of excretion of 3MH or of creatinine attributable to differences in nutritional status, of energy intake or to infection was observed. The linear relationship between the excretion rate of all three metabolites and body weight which was observed in recovered children was absent during acute infection. Nitrogen excretion was correlated (P less than 0.05) with the level of energy intake. The linear relationship between the rate of excretion of 3MH and creatinine and of 3MH and nitrogen was significantly closer in recovered than in ill children. The simultaneous effects of infection, underfeeding (and oliguria) on the rate of excretion of protein metabolites may be complex and contradictory. Our results suggest that the excretion of 3MH, creatinine and nitrogen is sustained during infection accompanied by negative energy balance. Disruption during infection of the relationship between the excretion of all three metabolites and body weight, and between 3MH and the other two metabolites suggests perturbation in protein metabolism at this time.