The long-lived 222Rn decay products 210Pb, 210Bi and 210Po have been monitored in the plumes of several vents at Mount Etna (Sicily) from May to October 1986. The results show that the four main craters of this volcano emit gases whose compositions are different from each other. The 210Bi/210Pb ratios for the plumes have similar mean values, (close to 25), which correspond to a degassing time of 1.5 to 2.7 days, according to the model of Lambert et al. (1985/86). In contrast, 210Po/210Pb ratios have very different mean values in each plume: 35 at the Voragine crater, 20 at the Bocca Nuova crater, and 14 at the South East crater. These figures enable us to calculate proportions of deep magma of 50%, 29% and 19% in the degassing cells of these craters respectively. Moreover, the SE crater appears to be a secondary degassing vent, not directly related to the main magma reservoir. The evolution of these ratios has been related to variations in volcanic activity. ¿ American Geophysical Union 1988