The distribution of excess 222Rn above the North Cleft segment of the Juan de Fuca Ridge were determined from 1988 to 1991 in order to obtain a better understanding of the temporal variability of heat flux from this ridge segment. Based on the inventories of excess 222Rn, the hydrothermal heat flux was estimated to be 2.7¿1.3 GW in 1990. The large uncertainty results from errors in calculating the standing crop of 222Rn, the difficulty in determining the 222Rn to heat ratio, and the problem of entrainment of low-temperature waters into the rising plume. The high-resolution sampling program carried out in 1990 permitted the mapping of apparent plume ages for different sections of the neutrally buoyant plume. About 3 km to the east of the ridge axis there is a sharp gradient in the apparent age of the plume, while to the west the plume appears to be younger. This plume age distribution could indicate a current flow to the northwest during the time this transect was sampled. The mean 222Rn/heat ratios for the high- and low-temperature vent fluid were 0.0017 disintegrations per minute (dpm) g-1¿C-1 and 0.0044 dpm g-1¿C-1, respectively. The extrapolated end-member concentrations of excess 222Rn in low-temperature vent fluids are a factor of 6 greater than the high-temperature fluids, based on the regressions of excess 222Rn activity and Mg concentration in North Cleft segment vent fluid samples.