There have been several reports suggesting that the assay of human placental lactogen(HPL) in maternal serum is valuable as a placental function test. The present study was undertaken to see the normal values of maternal serum HPL at different stages of normal gestation and its significance as an index of placental lfunction. One hundred and twenty six venous samples were randomly drawn from one hundred and fourteen normal pregnant women at various periods of gestation. For the determination of HPL in serum of normal pregnanl women, rapid radioimmunoassay had been performed using Phadebas HPL Test Kit and following results were obtained. 1. Maternal serum HPL levels increased progressively throughout pregnancy, and its regression equation according to week of pregnancy was as follows : Y=0.19898X-1.87133(r=0.88745, P<0.001, Y=value of serum HPL, X=week of pregnancy). 2. The initial determination of HPL in maternal serum was noted at six weeks but was generally present in minute amounts during the first trimester. The mean concentration increased gradually from only 0.03μg/ml at six weeks to 0.25μg/ml at 12 weeks. 3. From the second trimester onward the mean serum concetration of HPL rose steeply in a linear fashion until 38 weeks after which it gradually declined. The peak level of HPL determined in the prestudy was 6.23±1.664μg/ml at 38 weeks of gestation and prolongation of pregnancy beyond 38 weeks was found to be a fall of HPL levels to 5.02±0.944μg/ml at 42 weeks of gestation. 4. During the last trimester, there was greater variability in normal serum HPL levels and the lower levels of normal HPL formed a plateau just above 4μg/ml from the 34 weeks to the 42 weeks and only two out of total recordings taken from the 34 weeks onwards were below 4μg/ml. 5. The present results showed a relationship between mean HPL levels and placental weight or fetal weight but no correlation was found between mean HPL levels and maternal age, parity, or sex ef the fetus.