CHANCES in the carbon isotope ratio (delta(13)C) of atmospheric CO2 can be used in global carbon-cycle models(1-5) to elucidate the relative roles of oceanic and terrestrial uptake of fossil-fuel CO2. Here we present measurements of delta(13)C made at several stations in the Northern and Southern hemispheres over the past decade. Focusing on the highest-quality data from Cape Grim (41 degrees S), which also provide the longest continuous record, we observe a gradual decrease in delta(13)C from 1982 to 1993, but with a pronounced flattening from 1988 to 1990. There is an inverse relationship between CO2 growth rate(6) and El Nino/Southern Oscillation (ENSO) events which is not reflected in the isotope record. Thus, for the ENSO events in 1982, 1986 and 1991-92, we deduce that net ocean uptake of CO2 increased, whereas during La Nina events, when equatorial sea surface temperatures are lower, upwelling of carbon-rich water increases the release of CO2 from the oceans. The flattening of the trend from 1988 to 1990 appears to involve the terrestrial carbon cycle, but we cannot yet ascribe firm causes. We find that the large and continuing decrease in CO2 growth starting in 1988(6) involves increases in both terrestrial and oceanic uptake, the latter persisting through 1992.