The urinary excretion of histamine and its metabolites, methylhistamine, methylimidazoleacetic acid and imidazoleacetic acid, was measured under standardized dietary conditions in 24 women with normal pregnancies and in eleven patients with toxaemia of pregnancy. In addition, histamine metabolism was studied in five healthy women at delivery and in four other healthy pregnant women during treatment with aminoguanidine, which is an inhibitor of diamino oxidase (histaminase). A slight increase in the urinary excretion of methylimidazoleacetic acid was observed in normal pregnancy as well as in toxaemia of pregnancy compared to non-pregnant women. In two toxaemic patients and in one of the healthy subjects the urinary excretion of unmetabolized histamine was moderately increased. Despite the very high diamino oxidase activity in the plasma and in the uterus during pregnancy, there were no signs of altered catabolism of endogenous histamine in the pregnant women. Smoking increased the urinary excretion of the quantitatively dominant histamine metabolite, methylimidazolacetic acid, in pregnant subjects as it also does in nonpregnant subjects. The necessity of standardized dietary conditions in the study of histamine metabolism in man was again emphasized.