This study examines the acute effects in ECIG users of the two primary vehicles in ECIG liquids, propylene glycol and vegetable glycerin. Thirty experienced ECIG users will use an ECIG in five conditions that will differ only by the propylene glycol:vegetable glycerin ratio: 100:0, 70:30, 50:50, 30:70, and 0:100 (device voltage, heater resistance, liquid nicotine concentration, puff number, and interpuff interval all will be held constant). Plasma nicotine concentration, subjective effects, and puffing behaviors will be recorded in each condition.