1. Nicotine is of interest as a major constituent of cigarette smoke and as an additive to chewing gum used to help patients to stop smoking. 2. Specific pathogen-free rats were given an aerosol or injections of nicotine hydrogen (+) tartrate ( l -isomer) for 2 or 3 weeks at three doses and the number of secretory cells in the surface epithelium of airways was determined. 3. By aerosol, a dose giving 10 ng of nicotine/ml of plasma had no effect on secretory cell number. 4. By injection, nicotine had a ‘bi-phasic’ effect depending on dose: it reduced secretory cell number at plasma concentrations of 650 ng/ml of plasma or increased it at 2360 ng/ml of plasma. 5. Thus nicotine alone, given to rats at doses similar to those found in cigarette smokers and those who chew nicotine gum, does not cause secretory cell hyperplasia in the conducting airways of the rat.