We investigated the timing of transgene activation after fertilisation in Arabidopsis following crosses and using two transgenic promoters (from the AtCYCB1 and AtLTP1 genes). Using both a transactivation system and direct transcriptional fusion to drive beta-glucuronidase reporter expression, reciprocal crosses showed a lack of expression of the paternal components. This is consistent with a lack of paternal genome activity previously reported during early seed development in Arabidopsis [Viella-Calzada et al. (2000) Nature 404, 91-94]. However, transactivation experiments of the BARNASE gene gave evidence that at least some paternal loci retain transcriptional activity, though at a low level, during early embryogenesis.