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Groundwater from remote parts of the Middle Rio Grande Basin in north-central New Mexico has perchlorate (CO4-) concentrations of 0.12-1.8 mu g/L. Because the water samples are mostly preanthropogenic in age (028 000 years) and there are no industrial sources in the study area, a natural source of the ClO4- is likely. Most of the samples have Br-, Cl-, and SO42- concentrations that are similar to those of modern bulk atmospheric deposition with evapotranspiration (ET) factors of about 7-40. Most of the ET values for Pleistocene recharge were nearly twice that for Holocene recharge. The NO3-/Cl- and ClO4-/Cl- ratios are more variable than those of Br-/Cl- or SO4 (2-)/ Cl-. Samples thought to have recharged under the most arid conditions in the Holocene have relatively high NO3 Cl- ratios and low delta N-15 values (+1 per mil (parts per thousand)) similar to those of modern bulk atmospheric N deposition. The delta O-18 values of the NO3 (-4 to 0 parts per thousand) indicate that atmospheric NO3 was not transmitted directly to the groundwater but may have been cycled in the soils before infiltrating. Samples with nearly atmospheric NO3-/Cl- ratios have relatively high ClO4- concentrations (1.0-1.8 mu g/L) with a nearly constant ClO4-/Cl- mole ratio of (1.4 +/- 0.1) x 10(-4), which would be consistent with an average ClO4- concentration of 0.093 +/- 0.005 mu g/L in bulk atmospheric deposition during the late Holocene in north-central NM. Samples thought to have recharged under wetter conditions have higher (delta N-15 values (+3 to +8 parts per thousand), lower NO3-/Cl- ratios, and lower ClO4-/Cl- ratios than the ones most likely to preserve an atmospheric signal. Processes in the soils that may have depleted atmospherically derived NO3 also may have depleted ClO4- to varying degrees prior to recharge. If these interpretations are correct, then ClO4- concentrations of atmospheric origin as high as 4 mu g/L are possible in preanthropogenic groundwater in parts of the Southwest where ET approaches a factor of 40. Higher ClO4- concentrations in uncontaminated groundwater could occur in recharge beneath arid areas where ET is greater than 40, where long-term accumulations of atmospheric salts are leached suddenly from dry soils, or where other (nonatmospheric) natural sources Of ClO4- exist.