Nitric oxide (NO) is a gaseous free radical that reacts with O(2) in air and aqueous solution. NO donors have been widely used to circumvent the difficulties inherent in working with a reactive gas, but NO donors do not deliver NO at a constant rate for prolonged periods of time. Furthermore, some of the most commonly used NO donors produce additional, bioactive decomposition products. We designed and built an apparatus that allowed for the precise mixing of gaseous NO with air and the delivery of gas through sample vials at fixed rates. This experimental setup has the added advantage that continuous flow of gas over the sample reduces the buildup of volatile breakdown products. To show that this experimental setup was suitable for studies on the dormancy and germination of Arabidopsis thaliana seeds, we introduced vapors from water or sodium nitroprusside (SNP) into the gas stream. Seeds remained dormant when treated with water vapor, but gases generated by SNP increased germination to 90%. When pure NO was mixed with air and passed over dormant seeds, approximately approximately 30% of the seeds germinated. Because nitrite accumulates in aqueous solutions exposed to NO gas, we measured the accumulation of nitrite under our experimental conditions and found that it did not exceed 100 microM. Nitrite or nitrate at concentrations of up to 500 microM did not increase germination of C24 ecotype Arabidopsis seeds to more than 10%. These data support the hypothesis that NO participates in the loss of Arabidopsis seed dormancy, and they show that for some dormant seeds, exposure to exogenous NO is sufficient to trigger germination.