Using a radial immunodiffusion technique the levels of four serum proteins, albumin, transferrin, slow α1-globulin and γ2-globulin, were measured in normal rats to determine the effects on these proteins of age, sex, pregnancy and of the administration of oestrogen (mestranol) and progestogen (ethynodiol diacetate). Significant increases in albumin, slow α1-globulin and γ2-globulin were found with increasing age, and highly significant differences in albumin and slow α1-globulin occurred between male and female rats. During pregnancy significant increases in slow α1-globulin and transferrin were found but albumin and γ2-globulin concentrations fell. Oestrogen administration resulted in significant increases in slow α1-globulin and transferrin, but albumin and γ2-globulin were unaltered. In the progestogen-treated group the concentrations of all four proteins were unchanged.