Sections of Days 12 to 55 sheep chorion, embedded at low temperature in glycol methacrylate, were exposed to rabbit anti-ovine chorionic somatomammotrophin (oCS) to ascertain the distribution of oCS. This hormone was first detectable on Day 14, when the cytoplasm of each chorionic cell displayed a low level of fluorescence surrounding droplets which were shown to be lipid containing. At this time, all chorionic cells were uninucleate. By Day 28, binucleate chorionic cells had appeared but showed no binding of the oCS antiserum which was confined to a significant proportion of the uninucleate chorionic cells surrounding lipid droplets, as at Day 14. The same pattern of hormone distribution, although with reduced fluorescence intensity, was observed on Day 55; fluorescence indicative of antibody binding was seen only in some of the uninucleate chorionic cells. Hence, oCS was detected in chorionic tissue before the differentiation of binucleate cells (Day 14) and, at all stages, it was confined to the cytoplasm of specific uninucleate chorionic cells in close association with lipid droplets.