Recent human and animal research suggests that the startle reflex might serve as a psychophysiological indicator of the emotional valence of foreground stimulation. The present experiment was designed to evaluate the emotional effects of positive and negative odorant stimuli. We examined the effects of continuous hydrogen sulphide (H2S) and vanillin stimulation on the magnitude of the acoustic startle reflex (measured at the M. orbicularis oculi) and on ratings of subjective valence in 16 healthy subjects. In accordance with the view that odors have emotional qualities, we found that H2S, a presumed negative foreground stimulus, significantly enhanced the startle-reflex amplitude relative to neutral air stimulation, whereas vanillin, a positive foreground stimulus, tended to reduce the reflex amplitude compared with neutral air stimulation. Both odorant stimuli were rated as equally intense by the subjects, and heart rate and electrodermal activity were not affected differentially by the two odorants.