The role of endogenous inhibitors and growth substances in the regulation of bud dormancy of wood plants was investigated using mature trees, seedlings, isolated shoots, and aseptically cultured buds and shoots. The photoperiodic induction of dormancy was not mediated through changes in inhibitor β activity, or ABA levels of buds and leaves of Alnus viridis seedlings. Changes in the activity of the inhibitor β fraction of buds and leaves of field grown Alnus glutinosa occurred during autumn but were not correlated with the onset of dormancy. Exogenous applications of 10-4M ABA, 10-3M CCC, ABA + CCC, 10-3M C5, 10-3M C9 and 10-3M C10 to actively growing Alnus viridis seedlings did not result in the formation of dormant buds. Dormancy was not induced by pruning the root system of actively growing alder seedlings or by the treatment of isolated growing shoots with 10-4M ABA. ABA prolonged the dormancy of apparently dormant buds after their transfer to an environment favouring growth, but this effect was modified by species, time of year, presence and absence of leaves, and position of the bud on the stem. C8 and C9 prolonged the dormancy of Populus nigra Italica while C10 promoted bud burst in Salix alba/babylonica. CCC was ineffective in most species tested (Alnus, Populus and Salix) except when in combination with ABA. The use of aseptic shoots of Populus yunnanensis cultured in vitro as a bioassay for dormancy-inducing substances was found to be limited by the variable growth responses elicited. No dormancy-inducing substances were detected in extracts, including the inhibitor β fraction, of dormant buds or leaves or shoots. The presence of various concentrations of ABA, C5, C10, AMO 1618 and ABA + AMO 1618 in the medium did not cause the formation of dormant shoot morphology, although growth was inhibited by certain treatments. Results are discussed in relation to the inhibitor hypothesis of dormancy regulation.