Serial changes in 24-hour creatinine clearance were determined in 11 women during several menstrual cycles before conception, the conceptional cycle and the first trimester of pregnancy. During the menstrual cycle, a 20 per cent mean increase occurred between the week of menstruation and the late luteal phase. Following conception, this increase continued such that a 45 per cent mean increase was evident by the ninth week of gestation. In two women who aborted spontaneously, the change in 24-hour creatinine clearance in early pregnancy was not as great nor as sustained, this feature being apparent at least three weeks before any clinical abnormality. Possible reasons for the changes in glomerular filtration rate in early pregnancy and its clinical implications are discussed.