The effect of exogenous L-proline (1 mM, 10 mM, and 100 mM) on embryogenic cultures of larch, sitka spruce and oak subjected to environmental stresses was examined. Low temperature (4 degreesC) completely inhibited growth of the cultures and this was partially alleviated by the addition of proline. Our studies show that not only can cultures survive low temperatures, but are capable of active growth while the cold stress is still being applied. Growth was inversely related to [NaCl] with complete inhibition at 200 mM. Proline stimulated growth at all concentrations tested permitting growth with 200 mM NaCl even at low (1 mM) proline concentrations. Release of internal cellular potassium was inversely related to freezing temperature and this release was reduced by exogenous proline. These results for cultures of forest species are consistent with findings previously reported for deciduous herbaceous angiosperms and suggest that proline may have a role in protection of forest species from environmental stresses.