Plasma prolactin concentrations were measured daily throughout twenty-three menstrual cycles from regularly ovulating women. In five of the cycles conception occurred spontaneously. The frequency distribution of prolactin concentrations was calculated and an appropriate linear transform obtained. Means and 95% confidence limits were determined by inspection of the logarithmic probability plots and these gave mean concentrations (and ranges) of 302 (106-871) mU/1 for the nonconception cycles and 178 (58-550) mU/1 for the conception cycles. Distribution of prolactin concentrations was log-normal in all cases, although the population containing the conception cycles was distinct from the population of non-conception cycles. It is suggested that the normal range found in ovulating women is 80-800 mU/1 but that for conception to occur levels should be slightly lower and approximate to the range 60-600 mU/1. Thus the lower limit of normal is about 60 mU/1 and the upper limit about 800 mU/1. Any concentration over 800 mU/1 should be designated as hyperprolactinaemia.