The specific binding of <sup>125</sup>I-human prolactin (hPRL) was studied in different areas of the human brain. Particularly high binding affinity of the hormone was found in the choroid plexus and this tissue was therefore selected for further studies. The hippocampus, the hypothalamus and the pituitary were among other regions containing prolactin-binding sites. In the choroid plexus the amount of PRL receptors was significantly higher in females than in males and was also found in both sexes to decrease with age. The binding affinity of <sup>125</sup>I-hPRL to choroid plexus was 3.0 × 10<sup>9</sup> M-<sup>↑</sup>and the binding capacity was 10.3 pmol per mg protein. Following solubilization with Triton X-100 the PRL receptor fraction retained its hormone-binding properties and upon molecular sieve chromatography it behaved as a protein with a molecular mass of approximately 250,000. Cross-linking of <sup>I25</sup>I-hPRL to receptors from choroid plexus and subsequent sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis indicated a major hormone-binding unit of M<sub>r</sub> 44,000. This value is about 7,000 smaller than that reported earlier by us for the growth hormone receptors from the same tissue, following cross-linking to <sup>125</sup>I-human growth hormone (hGH). By affinity column chromatography a complete separation of the hPRL and hGH binding units was achieved. It was thus shown that in choroid plexus the binding sites for GH and PRL occur as discrete entities.