The distribution of helium isotope ratios in the various tectonic provinces of Turkey is examined through a synthesis of previously published data and the results of a recent survey along the North Anatolian Fault Zone (NAFZ) following the catastrophic 1999 earthquakes. The R/R-A values (where R = sample He-3/He-4 and RA = air He-3/He-4) cover a wide range from 0.05 to 7.87, and the mantle-derived helium is clearly identified in most locations, The mantle-derived component is high (> 50% of total He) in (a) regions of central and eastern Anatolia, both of which are associated with historically active volcanoes, and (b) the seismically active west-to-central segment of NAFZ. While the mantle contribution reaches a maximum at Nemrut volcano in eastern Anatolia-a region of dominantly compressional tectonics and with moderate enthalpy geothermal fields, it is relatively low (< 50%) in the western Anatolian extensional province even though this region has the highest geothermal potential. The average He-3/enthalpy ratios estimated for the different provinces suggest lithospheric stretching and rise of the geotherm as the major mechanism of high heat flow, yet limited intrusive activity in western Anatolia is suggested as the reason for the comparatively low He additions and consequently low (1.7 x 10(6) atoms/J) He-3/enthalpy ratios. A more extensive magmatic activity appears to be responsible for the greater input of both heat and helium in eastern and central Anatolia, with intra-province He-3/enthalpy variations (from 3.1 x 10(6) atoms/J in eastern to 0.25 x 10(6) atoms/J in central Anatolia) reflecting the ageing of hydro-magmatic systems. The relatively high He-3 concentrations in low enthalpy waters of northern Anatolia (2.8 x 10(6) atoms/J) are particularly significant since there is no evidence of volcanic activity associated with the strike-slip motion along the seismically active segment of NAFZ. Continuous monitoring of He-isotope compositions along NAFZ should lead to a better understanding of this apparent anomaly, as well as the relationship (if any) between He-isotope variations and seismic activities in this region. (C) 2002 Elsevier Science B.V. All rights reserved.