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Responses of denning grizzly bears (Ursus arctos) to noise associated with winter seismic surveys and small fixed-wing aircraft were studied on the north slope of Alaska during the years 1978-81. Changes in signal amplitude and collar temperature were monitored in 4 bears denned near seismic lines. Heart rates monitored by implanted transmitters, were measured in 1 of these bears and in a 2nd bear not subjected to seismic exploration activities. None of the bears left their dens as a result of seismic exploration activities. In undisturbed midwinter conditions, heart rates of 2 denned bears ranged 12-26 beats/min, but rose to 30-50 beats/min for brief periods at least once or twice in 24 hours. Signal amplitudes and collar temperatures, monitored in 1 bear, did not vary. During 3 days when seismic crews were working near 1 den, changes in signal amplitude and collar temperatures, accompanied by increases in heart rate to a maximum of 64 beats/min, indicated that the bear moved several times. Heart rates of 2 bears recorded during midwinter overflights were the same as those measured in midwinter from the ground in undisturbed conditions. About the time of emergence, heart rates were higher than those recorded in midwinter and during undisturbed resting behavior in mid-June.