The recent development of radioimmunoassay for prolactin has extended the depth and extent of studies of factors affecting prolactin levels in mammals and has permitted long-term studies to be made of changes in blood prolactin levels in individual animals. Although ovine prolactin has been widely studied in relation to its structure (Li, 1972) and biological activity (see Wolstenholme & Knight, 1972) and has been extensively used as a reference material, the serum prolactin levels of the sheep have not been widely investigated. Many of the early studies involved measurements of single samples obtained by venepuncture, a stressful procedure now known to affect prolactin release in the