Quantitative differences in transgene expression between independent transformants are generally ascribed to different integration sites of the transgene (position effect). The contribution of spatial and temporal changes in transgene promoter activity to these position-induced differences in transgene expression in planta are characterized, using the firefly luciferase (luc) reporter system. The activity of three different promoters (Cauliflower Mosaic Virus (CaMV) 35S, modified CaMV 35S and the promoter of an Arabidopsis thaliana Lipid Transfer Protein gene) was shown to vary not only among independent transformants, but also between leaves on the same plant and within a leaf. The differences in local LUC activity between leaves and within a leaf correlated with differences in local luc mRNA steady-state levels. Imaging of LUC activity in the same leaves over a 50 d period, shows that individual transformants can show different types of temporal regulation. Both the spatial and the temporal type of luc transgene expression pattern are inherited by the next generation. It is concluded that previously reported position-induced quantitative differences in transgene expression are probably an accumulated effect of differences in spatial and temporal regulation of transgene promoter activity.