We have previously shown that there was an estrogen-regulated developmental increase in low density lipoprotein (LDL) uptake by placental syncytiotrophoblasts during baboon pregnancy. To determine whether this reflected enhanced expression of the LDL receptor, the levels of LDL receptor messenger RNA (mRNA) were determined by Northern blot and reverse transcription-polymerase chain reaction in placental tissue obtained from baboons (Papio anubis) in early (days 58-64; pooled to yield 5 samples), mid- (days 97-110; pooled to yield 12 samples), and late (days 161-175; pooled to yield 15 samples) gestation (term = 184 days). Whole villous tissue and a trophoblast cell fraction isolated by 50% Percoll gradient centrifugation were analyzed. The latter cell fraction was equally comprised predominantly of syncytiotrophoblasts at early, mid-, and late gestation as determined by extensive immunocytochemical reactivity with antisera to syncytiotrophoblast-specific peptides. Tissues were extracted with guanidine isothiocyanate and 5 micrograms polyadenylated-enriched RNA hybridized to a 32P-labeled human LDL receptor complementary DNA (cDNA). A major 6.2-kilobase LDL receptor mRNA transcript was expressed in syncytiotrophoblasts and whole villous tissue, as determined by Northern blot. In the syncytiotrophoblast-rich cell fraction, LDL receptor mRNA levels, analyzed by Northern blot and autoradiodensitometry and expressed as a ratio of beta-actin, were similarly low in early (0.66 +/- 0.12 arbitrary units) and mid- (1.15 +/- 0.23) gestation, then increased to a level in late gestation (2.71 +/- 0.33) that was over 4- and 2-fold greater (P < 0.01) than that in early or midgestation, respectively. In contrast, in whole villous tissue, LDL receptor and beta-actin mRNA levels exhibited no consistent change or decreased slightly with advancing pregnancy, so that when corrected for beta-actin, LDL receptor mRNA levels were similar in early (1.53 +/- 0.33), mid- (1.44 +/- 0.16), and late (2.32 +/- 0.29) gestation. The unchanged levels of LDL receptor mRNA in whole placental villous tissue with advancing primate gestation may reflect potential villous tissue with advancing primate gestation may reflect potential confounding effects that nontrophoblast, e.g. vascular, components of the developing placenta may have on assessing trophoblast endocrine function in whole villous tissue. Amplification of trophoblast RNA by reverse transcription-polymerase chain reaction with LDL receptor primers generated a single cDNA product of approximately 258 base pairs.(ABSTRACT TRUNCATED AT 400 WORDS)