Geochemical fluctuations through time of the eruptions originating from Ischia island and from the Phlegrean Fields in the Campanian area have been analyzed in order to determine the relationships between the mechanisms responsible for compositional variations of the magmas and the fluctuations in eruptions. Geochemical fluctuations are derived from the detailed chronological and geochemical record of marine tephra in deep-sea sediment cores from the central Mediterranean Sea. Such fluctuations are marked by the repetition of similar compositional gradients during each eruptive cycle, as defined by a period of intense activity followed by a quiet eruptive period. Geochemical changes through time in both areas may be explained by magma evolution in a continuously crystallizing, tapped, and periodically refilled stratified magma chamber. Each eruptive and geochemical cycle starts by an influx of magma from depth into the shallow reservoirs, which leads to an increase in the frequency of the eruptions and maintains the activity in a particular volcanic area. This inflow may be controlled by changes of glacio-eustatic pressure, as suggested by its periodic occurrence every 24 kyr. ¿ American Geophysical Union 1993