Project Summary/Abstract The appeal and addiction potential of e-cigarettes (e-cigs) is influenced by constituents like nicotine (the primary reinforcer that produces sensory irritation) and cooling flavorants (which improve flavor and can reduce irritation). The tobacco marketplace is constantly introducing “disruptive innovations” in e-cigs, which may impact appeal and addiction potential. Two recent entrants into the marketplace that are widely available in e- cigs are the synthetic coolant WS-3 and synthetic nicotine, manufactured in the lab and is not from the tobacco plant. The FDA has regulatory authority over these constituents, but little is known about their impact. This proposal will use established human e-cig exposure models to conduct the first studies comparing synthetic WS-3 vs. traditional coolants (i.e. menthol) and synthetic nicotine vs. tobacco-derived among those who use e- cigs, to get consistent and reliable reports on user experiences. Aim 1 will characterize user experiences to nicotine e-liquids with the synthetic coolant WS-3 alone, when combined with menthol, and in comparison to menthol alone. After pilot testing to determine optimal WS- 3/menthol concentrations, adult users of e-cigs (50% with combustible tobacco use) will be exposed to three WS-3 concentrations (no WS-3, low, high) in combination with three menthol concentrations (no menthol, low, high; 9 total exposures) in a nicotine containing e-cig. Addiction potential and appeal will be examined with hypothesized similar increases in additional potential/appeal for menthol and WS-3 and additive effects when combined. Secondary and exploratory analyses with examine interactive effects of WS-3 and menthol and influence of predictors (e.g. age, sex, combustible tobacco use, nicotine dependence) on outcomes. Aim 2 will characterize user experiences to nicotine e-liquids with synthetic nicotine and menthol. In separate lab sessions, adult users of e-cigs (50% with combustible tobacco use) will be exposed to synthetic nicotine in tobacco and menthol flavor and tobacco derived nicotine in tobacco and menthol flavor in nicotine containing e-cigs. Addiction potential and appeal will be examined across conditions and repeated exposure to each condition will be used to examine withdrawal effects. We hypothesize synthetic nicotine will have greater appeal and addiction potential than tobacco-derived nicotine. Secondary and exploratory effects will examine the flavor and nicotine form interactions and influence of predictors (e.g. age, sex, combustible tobacco use, nicotine dependence) on outcomes. These pivotal and rigorous experiments will inform future research and regulatory needs to balance the impact of these synthetic constituents on initiation and harm reduction.