Cannabis use in pregnancy is associated with a range of obstetrical conditions. The molecular mechanisms underlying these effects have not been elucidated but are attributed to the actions of delta-9-tetrahydrocannabinol (Delta9-THC). In this study, concentrations of Delta9-THC equivalent to those found in the serum of cannabis users, i.e. approximately 20 microM, inhibited proliferation and activated a restricted tight transcriptional programme in the BeWo trophoblast cell line. Employing genome-wide expression profiling methods, we found that the pattern of gene expression differs from that described in the placenta of patients with fetal growth restriction (FGR), associated with either hypoxia or discordant dichorionic twins, or of patients with pre-eclampsia. It was also dissimilar to the patterns obtained from the transcriptome of other tissues, such as the mouse brain, treated with Delta9-THC. The expression of transcription factors, such as thyroid hormone receptor-beta1 (TRbeta1), and transcriptional co-repressors, such as histone deactylase 3 (HDAC3), was affected by Delta9-THC in a dose-dependent manner, whereby 15 microM Delta9-THC caused a 2.8-fold inhibition of TRbeta1 expression, but a 3.5-fold increase in HDAC3 expression. These data were confirmed by end-point RT-PCR analyses and underpin the observed Delta9-THC-induced inhibition of BeWo cell proliferation. Genes encoding for growth, apoptosis, cell morphology and ion exchange pathways were modulated by 15 microM Delta9-THC. This study may provide insight into the mechanisms underlying the effects of Delta9-THC and cannabis use upon placental development during pregnancy.