The effect of photoperiod and melatonin treatment on cold resistance and thermogenesis of quails was studied. The birds were acclimated for 8 weeks to short day (8L:16D) or long day (16L:8D) conditions, and 8 of 16 quails in each group were implanted with melatonin capsules. One group of quails was maintained outside in an aviary during winter. Oxygen consumption (VO2), body temperature (Tb, recorded with temperature transmitters) and shivering (integrated pectoral EMG) were recorded continuously, and samples of heart rate and breathing rate were picked up when ambient temperature was decreased stepwise from 27 down to -75 degrees C. Heat production maximum (HPmax), cold limit, lower critical temperature, basal metabolic rate (BMR) and thermal conductance were determined. The results show that short day, cold and melatonin treatment improved cold resistance and thermal insulation of quails when compared with quails acclimated to long day conditions. An increase in HPmax was induced only by melatonin treatment. The results suggest that the acclimatization of quails is under control of the pineal gland. The linear increase of shivering intensity with VO2 at moderate cold load shows that shivering is the primary source for thermoregulatory heat production in the quail. At Ta's below -40 degrees C shivering remained constant although VO2, heart rate and breathing rate continued to increase with increasing cold load. This could indicate the existence of a nonshivering thermogenesis in birds. Unlike to mammals, this non-shivering thermogenesis in birds would serve as secondary source of heat supporting shivering thermogenesis in severe cold.