Onion seeds, cvs. Aurora and Petrolini, were conditioned using distilled water or aqueous sotutions of potassium nitrate (KNO3) in the osmotic potentials of 0,1M, 0,3M and 0,5M using imbebition periods of zero (control), 24, 48 and 72 hours, at 20°C and with constant aeration. After each treatment the seeds were washed, superficially dried, weight and dried in stove with air circulation at 30°C until reach the initial moisture leveIs, the seeds were evaluated by germination test; germination in sub-optimal temperature test; germination in high temperature test; first count of germination test; speed of germination; cold test in paper rolls without soil and radicle length at nine days. The results showed that priming with aerated solutions of KNO3, 0.1M and 0.3M for 24 hours of soaking, was efficient in improve onion seed performance in both varieties. Germination under stress conditions, radicle length and germination speed increases in primed seeds, mainly in cv. Petrolini.