The urinary excretion of histamine and its metabolites methylhistamine (MeHi) and methylimidazoleacetic acid (MeImAA) was measured during the menstrual cycle in nine healthy women, one allergic woman and three non-pregnant women with anovulatory regular cycles. Simultaneous urinary analyses of luteinizing hormone (LH) and total estrogens were performed. The healthy women showed individual variations in the excretion of histamine, MeHi and MeImAA. This observation has been interpreted as an expression of minor individual differences in the catabolism of histamine. At midcycle an increase in the urinary excretion of histamine metabolites was sometimes evident and a statistically significant correlation could be established between MeHi and estrogen in urine. These results may support previous findings of histamine release by estrogens in uterine tissue but may also reflect an elevated histamine formation. The allergic woman excreted constantly increased amounts of histamine and its metabolites, especially when her allergic symptoms became aggravated pre-menstrually. She did not exhibit any change in MeImAA excretion at midcycle but the MeHi-excretion varied with the excretion of estrogens in the urine. The subjects with anovulatory menstrual cycles had low values of histamine and metabolites although within the normal variations.