3α-Hydroxy-5α-androst-16-ene and 5α-androst-16-en-3-one were isolated from the subcutaneous adipose tissue of swine and identified (Fig. la, b). These steroids, when excreted or secreted, could function as sex pheromones and initiate the olfactory communication stimulus regulating the mating behaviour of the domestic pig (Sus domesticus) (Signoret & du Buisson, 1961). Other researchers (Lerche, 1936; Craig, Pearson & Webb, 1962) have already established that a particularly strong sex odour (variously described as musk-, perspiration- and urine-like) was associated with the fatty tissues of the animal carcass. Even though the odoriferous, musksmelling androstenols have been isolated from the testis (Prelog & Ruzicka, 1944), adrenal (Gower & Ahmad, 1966) and salivary glands (Patterson, 1968b) of the pig, they have not been identified as components of fatty or adipose tissue. Stimulated by our theoretical prediction (Sink, 1967) that 3α-hydroxy-5α-androst-16-ene should be present, and Patterson's subsequent finding (Patterson, 1968a) of the oxidized form,