The aims of this study were to establish whether both the cystic fibrosis (CF) and multidrug resistance (MDR1) genes are expressed in the human placenta during development and differentiation. To study their pattern of expression during development, RNA was extracted from first, second and third trimester human placentas. To investigate differentiation, RNA was extracted from cytotrophoblast cells isolated from human term placentas and maintained in culture for 18, 66, 90 and 114 h and from the undifferentiated choriocarcinoma cell line JAr. Using the reverse transcriptase-polymerase chain reaction (RT-PCR) with gene specific, intron spanning primers, a cDNA product of 1 kb, as expected for CF expression, was detectable following 35 cycles of PCR from all RNA samples except those from JAr; in the latter a product was only detected in one sample out of four separate passages and this was only just detectable after 40 cycles of PCR. RT-PCR using MDR1 specific primers resulted in a product from all samples at 0.34 kb as expected if this gene is expressed. These results demonstrate that both the CF and MDR1 genes are expressed in the human placenta at all stages of development and differentiation, although the expression of the CF, but not the MDR1, gene appears to be much weaker in the undifferentiated JAr cells in comparison with cytotrophoblast cells.