The levels of cytoplasmic high-affinity oestradiol receptor sites were determined in the isthmus, isthmus-ampulla, ampulla and fimbria of human Fallopian tubes obtained from 23 women during the proliferative and secretory phase of the menstrual cycle and after menopause. Each segment was homogenized in 0.25 M sucrose and fractionated by differential centrifugation into an 800 g X 15 min nuclear pellet and a 100,000 g X 1 h cytoplasmic supernatant and pellet. The nuclear and cytoplasmic pellets were extracted with 0.01 M Tris/HCl pH 8.0 containing 0.25 M sucrose, 1 mM EDTA and 0.1 M KCl. The receptor levels were determined in the 100,000 g X 1 h supernatant (free receptors) and in the KCl extracts (bound receptors). Values are expressed per mug DNA. The concentration of receptors (total, free and bound) was significantly greater in the ampulla than in the isthmus in both phases of the menstrual cycle (P less than 0.001). The concentration of free receptors in the isthmus, isthmus-ampulla and ampulla obtained in the proliferative phase was not significantly different from that of the secretory phase. In the fimbria a significantly higher concentration (P less than 0.01) was observed in the proliferative phase. The bound receptors were significantly higher in all regions in the proliferative phase than in the secretory phase (P less than 0.01). As a consequence the concentration of total receptors was significantly greater along the tube in the proliferative phase (P less than 0.01). The levels of oestradiol in the nuclear pellet of each segment as measured by radioimmunoassay were significantly greater in the isthmic region (P less than 0.01) than in other regions of the tube. The inverse relationship between the concentration of oestradiol receptor sites and the nuclear levels of oestradiol along the Fallopian tube is discussed.