The stability and variation in the expression of foreign genes in transformed poplar (INRA clone 717 1B4, Populus alba x P. deltoides) is being investigated at different developmental stages (in vitro, greenhouse and field) and at various times of the year in stems and leaves. Different combinations of promoters (CaMV 35S and the cinnamyl alcohol dehydrogenase (CAD) promoter from Eucalyptus gunnii) and reporter genes (uid A, green fluorescent protein (GFP)) as well as an antisense coding sequence (poplar CAD) are being used to study the stability of both constitutively-controlled expression (35S promoter) and targeted, tissue- specific expression (CAD promoter). Initial results using GUS histochemistry indicate that a significant proportion of transformants show changes in expression levels (gain/loss) when grown in vitro or in vivo (greenhouse). Changes in the tissue specificity of expression have also been noted in plants transformed with the uid A gene under the control of the 35S promoter. No cases of gene silencing have been observed in moving from in vitro to in vivo conditions although in two transformants (35S-GUS) such a move has apparently resulted in the induction of gene expression. Fluorometric analyses are currently under way to confirm these observations and transformants are being characterised at the molecular level to determine copy number. Histochemical analysis of two 5-year-old transformed (35S-GUS) poplar trees growing under field conditions indicate that gene expression in these transformants is still controlled in a constitutive manner in different organs.