The present study, as a part of a broader investigation on protein-energy-malnutrition (PEM) in rural Zaire, was undertaken in order to clarify varying aspects of histidine metabolism in patients suffering from protein-energy malnutrition (PEM). Measurement of histidine and its derivatives was performed on blood samples, in urine and in stool ultrafiltrates of healthy rural controls, of PEM mothers and PEM children, before and during dietary treatment, and after histidine oral overloading. In stool ultrafiltrates, unusually high concentration of histidine and of three major catabolites (imidazoleacetic acid, imidazolelactic acid and imidazolepropionic acid) were identified, contrasting with undetectable levels of urocanic acid. On the contrary, very large amounts of urocanic acid may be recovered in the urine of PEM patients, especially in those characterized by very poor life expectancy. Urinary urocanic acid level thus appears as a very sensitive indicator of liver malfunction, in a manner similar to low plasma prealbumin and/or high A-alpha 1-gamma-glutamyltranspeptidase values. These findings point to the severe impairment of histidase and urocanase, two enzymes regulating the histidine catabolic pathway. Under appropriate refeeding, all biochemical abnormalities described in surviving PEM patients are reversible. The persistence of high urinary urocanic acid excretion indicates a worsening of the patient's condition.