INTRODUCTION: The vast majority of work on the physiological effects of nicotine in humans has been done in smokers or smokers trying to quit. Such studies can be confounded by tolerance, desensitization of receptors, or withdrawal. Because of these difficulties, there is still some dispute as to whether nicotine is proparasympathetic or prosympathetic in humans. To circumvent these difficulties, we assessed the effect of nicotine on autonomic function by measuring changes in heart rate variability (HRV) in nicotine-naive healthy subjects. METHODS: Twenty males and 20 females aged between 18 and 25 years received 4 mg oral nicotine lozenge or placebo. HRV was assessed in 15-min periods before, during, and after ingestion. RESULTS: There were no significant changes in any measure after placebo administration. During and after nicotine ingestion, heart rate increased to 78 +/- 2 beats per minute (bpm) from a baseline level of 75 +/- 2 bpm (p < .01). Nicotine significantly increased low frequency (LF; normalized units) from 66 +/- 2 at baseline to 70 +/- 2 at 15-30 min postingestion (p < .01) and decreased high frequency (HF; normalized units) from 28 +/- 2 to 24 +/- 1 (p < .01). LF/HF ratio was therefore substantially increased from 2.9 +/- 0.3 to 3.7 +/- 0.3 (p < .01). CONCLUSIONS: A single dose of 4 mg oral nicotine produces a significant reduction in HRV (i.e., a proportional decrease in high-frequency variability) in healthy young nonsmokers consistent with a reduced vagal activity. This has implications for nicotine replacement treatments aimed at cessation of smoking.