Cold, salt and frost are important environmental stresses in forest trees and may significantly reduce productivity. Elevated levels of proline are associated with these stresses and may help alleviate their effects. Transgenic hybrid larch (Larix leptoeuropaea) has been produced expressing a Vigna aconitifolia gene for pyrroline 5-carboxylate synthase, the rate-limiting step in proline synthesis. Embryogenic masses of hybrid larch were co-cultivated with Agrobacterium tumefaciens harbouring a binary vector expressing the gene. The integration of the gene into the plant genome was confirmed by Southern blot and by proline content analysis. There was an approximately 30-fold increase in proline level in transgenic tissue compared to non-transformed controls. The transgenic tissue lines were significantly more resistant to cold, salt, and freezing stresses and grew under conditions (200mM NaCl or 4 degreesC) that completely inhibited the growth of control cell lines. Our results indicated that introduction of proline over-accumulation into forest trees might be an effective strategy for ameliorating the effects of environmental stresses.