The relationship between mid-luteal plasma levels of progesterone and prolactin was studied in 75 women with regular menstrual cycles. Eighteen women had normal prolactin (mean 260 +/- 51.7 mU/l) and normal progesterone levels (mean 67 +/- 21.3 nmol/l). Thirty-nine women had elevated prolactin levels (mean 850 +/- 503 mU/l): progesterone levels were normal in all cases (mean 61 +/- 22.3 nmol/l). Eighteen women had evidence of luteal deficiency (mean progesterone 15.3 +/- 7.7 nmol/l); prolactin levels were normal in all cases (mean 243 +/- 106 mU/l). There was no correlation between plasma prolactin and progesterone levels.