The concentrations of immunoreactive somatomedin-C (Sm-C) in the sera of women with uncomplicated pregnancies were found to be progressively higher as the duration of gestation increased. Between 19 and 42 weeks of gestation, there was a significant correlation between serum Sm-C concentration and duration of gestation. A striking decline in Sm-C was observed following delivery. While the data are too preliminary to elucide the mechanisms for the raised Sm-C, the concurrent increase in Sm-C and human placental lactogen (hPL) raises the possibility that hPL may mediate the increment in Sm-C.