A total of 39 mice in groups of three was observed for 48 hours for locomotor activity and for aggression-related vocalizations during short (120 sec) but recurrent and aperiodic presentations of a 60-Hz field. The field was a sinusoid with a dominant magnetic component that averaged 1.7 mWb/m2 (rms) and its effects were analyzed in terms of strain of mouse, (DBA/2J pigmented versus CD/1 albinos), of nocturnal versus diurnal levels of activity and of trend. Highly reliable and persistent increases of activity of a field-locked sort were observed. Circadian periods, as expected, and strain of mouse were also reliable sources of variation: mice of both strains were more active nocturnally than diurnally; and while the CD mice had higher baselines of activity, the DBA mice were more reactive to the magnetic field. Data from control experiments with no mice, with sham-exposed mice and with anesthetized mice tend to exclude electrical artifact as the basis of the field-induced increases of activity. The influence of the field on the measure of aggressive behavior is only conjectural because of low and variable rates of vocalization, both during baseline measures and during periods of exposure to the field.