With progress of normal senescence of pea (Pisum sativum) foliage membranes free fatty acid: sterol ratios increase, this reflecting increment of phospholipolytic activity. The effect is simulated by exogenous application of phospholipase A2 in the presence of Ca2+ and calmodulin and inhibited by the calmodulin inhibitor fluphenazine. This trend is accompanied by an increase in membrane microviscosity and transition temperatures and, as evidenced by wide angle X-ray diffraction data, indicates a change from the liquid crystalline phase to a solid gel lipid bilayer configuration. Ethylene production, associated with plant senescence, closely follows the above biophysical changes. Membranes also manifest lipoxygenase activity, increasing with age and upon application of Ca2, calmodulin and phospholipase A2 the increment being inhibited by the more specific calmodulin inhibitor calmidazolium. Moreover, the polyunsatu-rated linolenic acid enhanced phospholipase A2-induced ethylene production, whereas the saturated stearic acid was ineffective. Results are interpreted as a Ca2+-triggered and calmodulin mediated phospholipase A2 activation whereby membrane integrity is impaired by the release of the 2-situated polyunsaturated fatty acids which in the freed form induce lipoxygenase and specific Ca2+ ionophoretic activity during the process of which an ethylene forming mechanism is activated.