We investigated the effects of zinc on the function and ultrastructure of endothelial cells in the case of a 48-day immobilization stress provoked in Chinchilla male rabbits (n=18) by placing them in metal hutches. Half of those rabbits (n=9) received an daily oral supplement of zinc at a dose of 0.3 mg/kg body weight (in the form of zinc acetate). The control rabbits had no intervention and received no supplement of zinc. The relaxation of smooth muscles from thoracic aorta as mediated by acetylcholine at concentrations from 10(-8) mol/L to 10(-4) mol/L was determined in isometric regime. Responses were expressed as the percentage of relaxation to prostaglandin F2alpha (2.10(-5) mol/L)-induced precontraction. The ultrastructure of endothelial cells was evaluated by electron microscopy. The level of total cholesterol and zinc in the blood serum was determined by an enzymatic method and by atomic absorption spectrometry, respectively. In rabbits receiving no zinc supplement, the relaxation of smooth muscles under the influence of acetylcholine concentrations from 10(-8) mol/L to 10(-4) mol/L was significantly (P < 0.05-0.01) lower than in rabbits receiving a supplement of zinc and lower than in control rabbits. Also, in the rabbits not receiving the zinc supplement, the level of total blood serum cholesterol was increased, but the concentration of zinc decreased. In rabbits receiving the zinc supplement, the contractility of the smooth muscles effected by acetylcholine did not change as compared with control rabbits, and we found a normal structure of endothelial cells and a normal level of total cholesterol and zinc in their blood serum. Thus, zinc played an important role in the maintenance of the normal ultrastructure and function of the endothelial cells in the rabbits receiving zinc under immobilization stress.