Natural perchlorate is believed to be of atmospheric origin, yet no systematic study has been conducted to evaluate perchlorate deposition rate and possible seasonal or spatial variations. This study evaluated perchlorate concentrations in weekly composite wet deposition samples acquired through the National Atmospheric Deposition Program from 26 sites across the continental United States, Alaska, and Puerto Rico for a 1-3year period. Perchlorate concentrations varied from <5ng/L to a high of 102 ng/L with a mean of 14.1 ± 13.5 ng/L for the 1578 total samples. The annual perchlorate flux by site ranged from a low of 12.5 (TX) to 157 mg/ha-year (NE) and averaged 65 ± 30 mg/ha-year for all sites. Perchlorate concentrations and flux in wet deposition were generally highest in May-August declining to lows in December-February. Average annual perchlorate flux was correlated (r > 0.5; p < 0.001) with Ca 2+, K +, NH 4 +, N0 3 -, Cl -, and S0 4 -2. Wet deposition rate of CI0 4 - in the conterminous United States (excluding Alaska, Hawaii, and Puerto Rico) while diffuse, represents a potential annual net mass flux of 51 000 kg, a value comparable to the estimated annual environmental releases from other known CI0 4 sources.