An ethosecretogram, which illustrates the phase relation between animal behavior and plasma hormone levels, was constructed in conscious rats. A catheter was placed into the right atrium in male rats 2 days before blood sampling. Plasma corticosterone levels were monitored individually through the catheter every 5 min over a period of several hours under freely moving conditions on an Animex, which records spontaneous locomotor activity. The results showed that the bursts of locomotor activity were temporally correlated with those of plasma corticosterone level, the peak of the former preceding the hormone peak by 15 min. Regarding the type of locomotor activity, feeding and grooming were significantly correlated with the subsequent hormone elevation, whereas drinking was not. In a separate series of experiments, effects of feeding and drinking on plasma corticosterone levels were examined in conscious rats that had been fasted or water deprived for 24 or 48 h. In both cases, feeding induced a more pronounced elevation than drinking, although locomotor activity counts per se increased more sharply in the latter. It is concluded that in the ethosecretogram specific types of locomotor activity such as feeding and grooming induce a phase-locked elevation of plasma corticosterone under freely moving conditions.