The isolation and identification of rat prolactin (Kwa, 1965; Kwa, van der Bent & Prop, 1967) made it possible to develop a radioimmunoassay method for the estimation of prolactin in plasma and in pituitary tissue (Kwa & Verhofstad, 1967). Results are now given for plasma levels of prolactin, expressed as ng./ml. in rats of the highly inbred strains U and R, Amsterdam and the F1 hybrids (R x U) during their oestrous cycle, using the rat prolactin prepared by the above mentioned method as standard (15·6 i.u./mg.). Daily vaginal smears were taken from the age of 10 to 12 weeks from rats maintained under controlled illumination (14 hr. light, 10 hr. darkness). Five rats of each genetic constitution were chosen; all had regular 5-day cycles. Blood samples were collected daily between 14.00 and 15.00 hr. from the occular venous plexus. The samples were used in the radioimmunoassays at the