Groundwater samples from the Lincolnshire Limestone have been analysed for tritium, radiocarbon, and the stable-isotope ratios 13C 12C, 18O 16O and D/H. The age of the water increases in a downgradient direction below overlying confining deposits and reaches a maximum age greater than 25,000 years within 15 km of the outcrop. The ?? 13C ratio ultimately attains the unusually low negative values of -2??? in a downgradient direction; this is approaching that of the aquifer matrix which is +2.35???. The reason is believed to be exchange of carbon between the groundwater and the matrix by a continuous process of precipitation and further solution of calcium carbonate. The ?? 18O and ?? D ratios imply that recharge of the aquifer during the late Pleistocene took place in the spring and autumn rather than the winter as at present. The data are interpreted by assuming that movement of water through the saturated zone is a combination of flow in fissures and "piston flow" through the micro-fissures and pores of the rock. The mechanism of water movement in the saturated zone is dominated by relatively rapid flow in fissures, but the fissure flow includes a contribution of much older water from "intergranular" storage which enters the fissures from the rock matrix by pressure differentials in the fissure distribution system and by diffusion. The distribution of water of different ages in the aquifer is closely related to recent groundwater abstraction patterns. ?? 1977.