Plasma steroid concentrations measured in post-menopausal women with breast cancer showed a close correlation between the various androgens. The postmenopausal women exhibited a correlation between estrogen receptors (ER) and “arterial” 17α-hydroxyprogesterone levels in the breast. ER levels expressed as a function of plasma (arterial) steroid (P/17 P) molar concentration ratio were distributed within a hyperbola, suggesting the existence of a control mechanism for breast cell ER levels.