Metal oxide nanowires have been deposited on 100nm SiO2/Si substrates and SnO2 and ZnO single nanowire devices have been fabricated by electron beam lithography technique. Gas sensing tests have been carried out, in order to detect ammonia, CO and NO2 under standard humidity conditions. The devices have been also tested in either dark condition or exposed to ultraviolet light at room temperature. SnO2 nanowires are able to detect ammonia at concentration of the order of few ppm also in dark conditions, they are sensitive to NO2 and cannot detect CO. ZnO devices show good sensitivity to 5ppm of ammonia in 30% relative humidity atmosphere. The samples have been mounted on a TO-39 case in a field effect transistor configuration in order to study the influence of the back-gate voltage on the single nanowire devices gas sensing performances.