INTRODUCTION Since the pioneer studies of Hogben and his associates (1922a,b,c, 1923, 1924, 1930, 1931, 1936) and Etkin (1935, 1941, 1943), the pars intermedia of the pituitary gland and its hormone secretion, melanocyte-stimulating hormone (MSH; Intermedin) has been thoroughly investigated in the Amphibia and to a lesser extent in fish and reptiles. The role of this gland in pigmentary control is well established in these species and has been adequately reviewed already (Parker, 1948, 1950; Waring & Landgrebe, 1950; Pickford & Atz, 1957; Etkin, 1962, 1967; Waring, 1963; Legait, 1964; Landgrebe & Mitchell, 1966; Jørgensen, 1968). In comparison, the pars intermedia of mammals has received less attention. There has been a number of survey articles devoted to selected aspects of the mammalian gland, e.g. the comparative anatomy (Legait, 1964; Hanström, 1966; Wingstrand, 1966a) and the microscopic anatomy (Purves, 1966; Wingstrand, 1966b), but no comprehensive review of this