A homogeneous group of 34 subjects exposed to occupational vibratory stress were studied to determine their vasodilatory response to methacoline. The methacoline was administered by iontophoresis. Methacoline is an acetylcholine derivate with the same pharmacological properties of acetylcholine. The 34 study subjects were divided into 3 groups according to their plethysmographic response to a preliminary cold test: group 1, had a typical Raynaud's response; group 2 had normal response to cold, and group 3 had an "intermediate" response. The subjects in group 1 did not respond to methacoline with vasodilation, suggesting endothelial damage at the arterial and arteriolar level, those in group 2 responded with clear vasodilation, and those in group 3 had an intermediate plethysmographic response to methacoline--modest, slowed vasodilation. Based on the results obtained, the authors believe that the true pathogenetic mechanism is an alteration in the vasomotor stability. This change in circulatory homeostasis occur after a vasoconstrictive stimulus, due to vascular endothelium disfunction, in its turn damaged by vibratory microtrauma. This would be the basis for diminished autonomic vascular regulation, mediated by EDRF at the arterial and arteriolar level. It is also probable that the pathogenesis of Raynaud's syndrome in workers exposed to physical or chemical stress has a similar mechanism.