We measured the corticosterone and dihydrotestosterone steroid binding capacities of corticosteroid-binding globulin (CBG) and sex steroid plasma binding protein (SBP) throughout pregnancy in seven Macaca nemestrina. The nSBP binding capacity rises significantly by 28 days of pregnancy, remains elevated through 56 days, tends to fall through midpregnancy, and then decreases dramatically to very low values 130-153 days after conception. In contrast, nCBG rises slightly (1.2-fold) in early pregnancy, is elevated in midpregnancy, but is similar to nonpregnant values during the last trimester of pregnancy. Our data indicate that factors controlling the plasma concentration of nCBG and nSBP are not identical. The results are in sharp contrast to those observed in human pregnancies in which both hSBP and hCBG are increased severalfold during pregnancy, including at term. Because the changes of nCBG and nSBP during pregnancy do not paralle the changes observed in human beings even though their steroid binding characteristics are similar, the M. nemestrina may serve as a valuable model for defining the physiologic function of SBP and CBG and the mechanism(s) controlling their concentrations in blood.