A 20-fold increase in the relative expression of the hepatic GH-binding protein (GHBP)-encoding message between nonpregnant and 17-day pregnant mice was found. The hepatic GH receptor (GHR)-encoding message increased 8-fold between nonpregnant and pregnant mice. The increase in both messages began on day 9 of pregnancy. The steady state level of the GHBP-encoding message continued to increase steadily until day 17 of pregnancy; however, by day 13 of pregnancy, the steady state level of the GHR-encoding message reached a plateau that continued to day 17. The ratio between the GHBP- and GHR-encoding messages gradually increased during the second half of pregnancy, reaching a maximum on day 17. There was a 10- to 16-fold increase in GH-binding capacity in liver microsomes and a 30- to 50-fold increase in serum GH-binding capacity between nonpregnant and late pregnant mice. The increase in hepatic GH-binding capacity began on day 9 of pregnancy and reached a plateau on day 11, which was maintained until the end of gestation. The increase in serum GH-binding capacity began on day 9 of pregnancy and continued to increase until day 17. No significant change in mouse (m) GHR (mGHR) or mGHBP affinity constants were observed between nonpregnant and pregnant mice; however, the mGHR had a 20-fold greater affinity for mGH than did the mGHBP. The serum GH concentration increased in the second half of pregnancy. The GHBP-bound and the free fractions of GH during pregnancy were predicted. While the bound fraction of GH is predicted to parallel the total GH concentration measured by RIA, the concentration of free mouse GH remains unchanged during pregnancy.