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Twenty-two children were studied as inpatients at a Nigerian Hospital. They were divided into four groups on the basis of weight for age: I, adequately nourished, acutely infected; II, moderately under weight, acutely infected; III, malnourished, chronically infected; IV, malnourished, uninfected. Urinary nitrogen excretion was highest in group I and lowest in groups III and IV. Urinary creatinine was highest in group I, but did not differ significantly in groups II, III and IV. The excretion of 3-methylhistidine closely paralleled that of creatinine. It is suggested that the high rates of creatinine and methylhistidine excretion in group I resulted in part from destruction of muscle. Rates of whole body protein turnover were measured by administration of a single dose of [15N]glycine with measurement of the excretion of 15N in urinary NH3 for the next 9 h. Rates of protein synthesis and breakdown were very high in infected children of groups I and II. Although rates were lower in the malnourished groups, in infected children of group III they were nearly twice as high as in the uninfected group IV. The net balance of protein (synthesis minus breakdown) was negative in group I, less negative in group II, zero in group III and positive in group IV. Repeat measurements in group I during recovery from infection showed a decline in rates of excretion of nitrogen, creatinine and 3-methylhistidine. Rates of protein synthesis and breakdown declined and the protein balance became less negative, but these changes were not statistically significant. Multiple regression analysis of the results of all groups taken together showed independent contributions to rates of protein metabolism from infection and nutritional state, especially plasma albumin. It was concluded that infection caused a rise in protein breakdown which was larger than the concomitant rise in synthesis, leading to net loss of protein, and that these responses were reduced by malnutrition.