To assess the possible role of placental growth hormone (GH) in fetoplacental growth, we measured placental and pituitary GH (GHN) in maternal plasma by means of two RIA using two MAb (5B4 recognizing both placental GH and GHN, and K24 recognizing only GHN) during pregnancy. IGF-I also was measured by RIA in the same samples after extraction. A transverse study of 186 samples obtained between 8 wk of amenorrhea (WA) and term confirmed the reported rise in GH immunoreactivity with 5B4 after 24 to 25 WA from 12.3 +/- 2.0 mU/L (mean +/- SEM) to a plateau of 27.5 +/- 3.4 mU/L at 34 to 35 WA together with the decrease in GHN to undetectable levels by 24 to 25 WA. IGF-I levels increased from 164.0 +/- 44.6 micrograms/L at 24 to 25 WA to 331.6 +/- 63.6 micrograms/L at term. A longitudinal study of 31 normal pregnant women confirmed this hormonal pattern and the reported placental GH plateau after 35 WA. A drastic decrease in placental GH was observed with the onset of labor (from 26.9 +/- 2.1 to 2.7 +/- 1.1 mU/L), whereas the decrease in IGF-I was not significant (from 212.9 +/- 26.5 to 162.4 +/- 16.9 micrograms/L).(ABSTRACT TRUNCATED AT 250 WORDS)