Investigations were made on the influence of negative ionization of the air, in time doses of 10, 20 and 30 min. per diem, on motor activity in Syrian hamsters in photoperiods LD 16:8 and 8:16. The experimental animals were subjected to additional ionization of the air, obtained with on ionizer, of 14.000 negative light ions output per 1 cm3 air. Negative ionization applied in the photoperiod LD 16:8 causes a rise in activity of hamsters at doses of 10, 20 and 30 min. per diem. In the photoperiod LD 8:16, with a short day and a lengthened night, negative ionization applied for 10 or 20 min. per diem decreased motor activity in hamsters, but at 30 min. per diem decreased motor activity in hamsters, but at 30 min. per diem increased this. The circadian rhythm pattern of hamsters does not undergo change on the photoperiods and negative ionization of the air applied.