Gross-alpha and gross-beta, activities of eight well and five tap water samples taken in istanbul were determined. Ra-226, Rn-222 Pb-214, Bi-214, K-40 Cs-137 activity concentrations in four lake, four sea water, one snow and one rain water samples were also analyzed in order to determine their radioactivity. The results obtained showed that, in general, natural activities in drinking water samples did not exceed WHO and ITS guidelines. In sea and lake water, four samples were over WHO and TSI guidelines. Concentrations ranging from 0.007 to 0.04 Bq l(-1) and from 0.02 to 0.1 Bq l(-1) were observed for drinking water and the gross-alpha and gross-beta activities, respectively. For all samples the gross-beta activities were higher than the corresponding gross-alpha activities. In order to evaluate the annual effective dose equivalent of ingestion of these waters, a conservative dosimetric calculation was carried out using dose conversion factor suggested by the ICRP. An average annual effective dose equivalent of 0.84 mu Sv y(-1) for Ra-226 was calculated. (C) 2000 Elsevier Science Ltd. All rights reserved.