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Background. Bright light and high-density negative air ion exposure are efficacious for winter and non-seasonal depression compared with a low-density negative ion placebo. Similarly, auditory stimuli improve mood in clinical populations. This study compared the short-term effects of bright light, an auditory stimulus, and high- and low-density negative ions on mood and alertness in mildly depressed and non-depressed adults.Method. One hundred and eighteen subjects, 69 women and 49 men (mean age±S.D., 19·4±1·7 years), participated once across the year. Subjects were randomly assigned to one of four conditions: bright light (10000 lux; n=29), auditory stimuli (60 dB; n=30), or high-density (4·5×1014 ions/s flow rate; n=29) or low-density (1·7×1011 ions/s; n=30; placebo control) negative ions. Exposure was for 30 min on three consecutive evenings between 1900 and 2100 hours. Mood and alertness assessments, using standardized scales, occurred before, and 15 and 30 min during exposure. The Beck Depression Inventory classified subjects as depressed (10; n=35) or non-depressed (<10; n=83).Results. The three active stimuli, but not the low-density placebo, reduced depression, total mood disturbance (a global affect measure) and/or anger within 15–30 min. Neither testing season nor degree of depressive symptoms affected response to stimuli.Conclusions. The auditory stimulus, bright light and high-density ions all produced rapid mood changes – with small to medium effect sizes – in depressed and non-depressed subjects, compared with the low-density placebo, despite equivalent pre-study expectations. Thus, these stimuli improve mood acutely in a student sample, including a subset with depressive symptoms. [ABSTRACT FROM AUTHOR]