The effect of tryptophan on immobility in Porsolt's swim test was studied in male NIH Swiss mice. Preexposure to a swim or fight-stressor was included in the design. Doses of tryptophan (0, 12.5, 50, 75, 100, 125 and 200 mg/kg) were administered intraperitoneally 60 min prior to the swim test. In the nonstressed mice tryptophan had an U-shaped dose-response relationship: immobility in the water was dose-dependently shortened after doses from 0 to 100 mg/kg, whereas after 125 and 200 mg/kg tryptophan the immobility times did not differ from the values obtained after a saline injection. Preexposure to a swim- or fight-stressor did not make mice more sensitive to the effects of tryptophan. Tryptophan (0-300 mg/kg) had no effect on exploratory behavior or locomotor activity in the holeboard, suggesting that sedation was not a factor in the swim test results. The findings suggest that tryptophan has antidepressant -like properties in Porsolt's swim test.