Immunoreactive erythropoietin was estimated in samples of serum collected during the follicular and luteal phases of the menstrual cycle in seven women and in sera collected sequentially throughout normal pregnancy in these and three other women. Estimates of serum erythropoietin during the follicular and luteal phases of the menstrual cycle were almost identical. During pregnancy in all 10 subjects serum erythropoietin increased at some time after 8 weeks gestation. These changes were not related to other endocrine values, to indicators of haematological and renal function or to infant birthweight except that changes in serum erythropoietin and placental lactogen were related (P less than 0.00001). In one other woman treated over 17 weeks of pregnancy with continuous intravenous infusion of salbutamol, erythropoietin levels were within the range found in normal pregnancies.