Adiponectin, a fat-derived factor, is downregulated in insulin resistance and obesity; insulin resistance has been demonstrated during late pregnancy in both humans and in rodents. The present study examines the physiological change of adiponectin gene expression as well as the circulating levels of adiponectin during pregnancy. We examined the relative quantity of adiponectin mRNA produced in the adipose tissues of pregnant compared to virgin mice. We also measured serum adiponectin levels and parametrial adipocyte size in mice throughout pregnancy. Adiponectin mRNA was significantly reduced by 74 +/- 8 % and 63 +/- 4 % at days 15 and 18 of pregnancy, respectively, compared to virgin mice. Serum adiponectin concentration decreased on days 15 (30.7 +/- 8.5 microg/ml) and 18 (27.9 +/- 8.7 microg/ml) of pregnancy, and the values were significantly lower than that of virgin mice (56.8 +/- 6.6 microg/ml). Parametrial adipocytes from mice on days 15 and 18 of pregnancy were significantly larger than in virgin mice or during early pregnancy. Fat-cell size was closely correlated to degradation of adiponectin gene expression and serum adiponectin levels. These results suggest that changes of adiponectin expression affect metabolic status in pregnant mice.