The 1-aminocyclopropane-l-carboxylate oxidase (ACC-ox) reaction of cultured sycamore cells was previously shown to be stimulated «in vivo» by fusicoccin and plasma-membrane (PM) red-ox activity. In this paper we report that a stimulation of «in vivo» ACC-ox was also obtained by administration of isobutyric acid (IBA) and staurosporine to the cultures. A comparison of the characteristics of the stimulation by all of these different chemicals has shown that: 1) as with fusicoccin-induced, the red-ox-induced ethylene formation was inhibited by high osmotica (0.6 mol/L mannitol); 2) on the contrary, the stimulation of ethylene production, either by IBA or by staurosporine, was insensitive to plasmolyzing mannitol; 3) ethylene overproduction, sensitive to mannitol, was also inhibited by vanadate, an inhibitor of the PM proton pump, and by tetraethylammonium (TEA), an inhibitor of PM K + channels; and 4) any stimulation insensitive to plasmolyzing mannitol was not inhibited either by vanadate or by TEA. The relationship between the action of mannitol and the ACC-ox activity was also investigated. A gende rehydration of the mannitol-treated cells led to the recovery of a large part of the stimulation, while this did not occur if the cells were subjected to a rapid rehydration in the cold which leads to an osmotic shock. The data indicate that two differently regulated types of ethylene production are present in sycamore cells and that one of these is largely dependent on the biochemical and physical status of the cell membranes.