The objective of the present study was to determine whether, in addition to the onset of chorionic somatomammotropin (CS) production previously shown to result from the morphological differentiation of cytotrophoblasts into syncytiotrophoblasts, there is a further developmental increase in the capacity of syncytiotrophoblasts to produce CS with advancing stages of baboon pregnancy. Placentas were obtained from baboons in early (days 48-62), mid (days 97-110), and late (days 161-175) gestation (term = 184 days), and CS messenger ribonucleic acid (mRNA) and protein levels were determined in a syncytiotrophoblast-rich cell fraction isolated by Percoll gradient centrifugation. CS mRNA levels in syncytiotrophoblasts, expressed as a ratio of beta-actin, exhibited a progressive increase from early (0.04 +/- 0.04 relative arbitrary units) to mid (2.37 +/- 0.33; P < 0.001) to late (3.66 +/- 0.39; P < 0.05) gestation. Levels of the 22-kDa CS protein were very low on days 48-55 (0.83 +/- 0.09 arbitrary units), increased 10-fold (P < 0.001) on days 57-60 (8.11 +/- 0.68), and increased (P < 0.001) to a maximum of 14.58 +/- 0.58 near term. CS mRNA levels in whole placental villous tissue increased (P < 0.05) between early (0.89 +/- 0.48) and mid (2.97 +/- 0.47) gestation, then remained constant. CS protein exhibited a similar increase (P < 0.001) in villous tissue between early (2.32 +/- 0.40) and mid (6.07 +/- 0.24) gestation, then remained constant. The increase in mRNA and protein levels of CS in the placenta was accompanied by a progressive (P < 0.001) rise in serum CS. We conclude that in addition to the morphological differentiation of cytotrophoblasts into syncytiotrophoblasts that has been well established to result in the onset of CS biosynthesis, villous syncytiotrophoblasts undergo functional/biochemical differentiation thereafter, manifested as an increase in the capacity for the synthesis of CS.