Placental trophoblast syncytialization is a unique biological process. We have studied the time course of this process using DNA microarray in a cell model of syncytialization (the cytotrophoblast cell line BeWo following increased intracellular cAMP by forskolin). Total RNA was extracted from BeWo cells and labelled-cRNA target was then hybridized to a specific oligonucleotide probe set containing probes to over 12?000 human transcripts. Detectable levels of signal were found on average for 44 per cent of the total number of genes assayed. The correlation coefficient for the level of expression of independent replicates was #10878;0.99. The mRNA expression profile of specific genes analysed by microarray correlated quantitatively well with that analysed by reverse transcription-polymerase chain reaction and with protein secretion. In the absence of forskolin there are relatively few changes in gene expression (reaching a threshold of two fold); in the presence of forskolin there are a substantial number of changes. By clustering the patterns of altered gene expression at least ten groups could be extracted. Seven of these clusters involved increased gene expression and three decreased expression. Each cluster has been categorized by gene ontology (confining the analysis to genes with 'known' function). Among the genes with increased expression following forskolin treatment were many required for cellular communication (such as placental specific peptide hormones) and metabolism (such as cholesterol side chain cleavage enzyme). Several genes known to be involved in cell adhesion and fusion have markedly changed expression levels very early following forskolin exposure, thus preceding morphological fusion of BeWo cells. Further analysis of this data and expression profiling in general will be able to contribute to understanding the functional basis for the formation of the placental syncytiotrophoblast.