The possibility that a sub-clinical deficiency of the trace element selenium might exist in a sample of the British population was examined by giving a selenium supplemented for 5 weeks. Using a double-blind crossover design 50 subjects received either a placebo or 100 micrograms selenium on a daily basis. On three occasions they filled in the Profile of Moods state. Mood did not change when taking the placebo, whereas when taking the selenium the subjects reported a substantial improvement after both 2.5 and 5 weeks.