In situ hybridization was used on both routinely processed paraffin embedded tissue sections and cryosections to study the synthesis of the basement membrane (BM) proteins laminin and type IV collagen in early human placentas from 8 to 11 weeks of gestation. Complementary DNA and RNA probes coding for the pro alpha 1 (IV) chain of human type IV collagen and the B1 chain of human laminin were used to detect respective mRNA. The clusters of cytotrophoblastic cells (i.e., cytotrophoblastic cell columns) contained cells that expressed both laminin and type IV collagen mRNA. Nearly all the stromal cells of developing villi also expressed mRNA for these proteins, type IV collagen mRNA levels being more pronounced than those of laminin. Conversely, the cytotrophoblastic cells of the villous trophoblastic epithelium synthesized mRNA for laminin more actively than that for type IV collagen. Decidual cells, endometrial stromal cells, and cells in the wall of spiral arteries all expressed mRNA for both laminin and type IV collagen. The results of in situ hybridization were found to correlate well with immunohistochemical stainings with specific antibodies to laminin and type IV collagen. The trophoblastic cells of the columns and the stromal fibroblasts are shown to contribute significantly to BM protein synthesis in the developing human placenta. The abundance of BM proteins may indicate their significance in guiding tissue organization in the placenta.