SUMMARY: A solid phase-antibody system is described for the radioimmunoassay of ovine prolactin. The sensitivity of the method is equal to 0·5 ng of prolactin/ml. Prolactin was measured in the serum of pregnant and lactating ewes; the levels increased at 5 weeks before parturition and then decreased during the following 3 weeks. A second peak of prolactin was observed at parturition and then the levels gradually decreased during the first 3 weeks post partum.