1. Renal sympathetic nerve activity, heart rate and arterial blood pressure were simultaneously measured in thirteen awake cats before, during and after eating which was evoked by presenting food for a period of 10-15 s. 2. With food presentation, eating behaviour occurred in 93% (191) of 205 trials, and renal sympathetic nerve activity significantly increased in 65% of the 191 trials. On the other hand, in many of food presentation trials when no eating occurred, or with presentation of an empty food box, renal sympathetic nerve activity did not change significantly. 3. Eating started 1-8 s after the food presentation. The increase in renal sympathetic nerve activity was closely related to the beginning of eating but not to the onset of food presentation. Renal sympathetic nerve activity and heart rate increased with a slight time lag of 0.5-1.5 s from the onset of eating, whereas an increase in arterial blood pressure followed the onset of eating by 5.5 s. After the beginning of eating, renal sympathetic nerve activity, heart rate and arterial blood pressure increased at a maximum of 61 +/- 18% (mean +/- S.E. of mean), 26 +/- 4.0 beats/min, and 17 +/- 4.9 mmHg from the control values at 1.0, 5.5 and 11.5 s, respectively. 4. Cardiac-related grouped discharges of renal sympathetic nerve activity, which were observed at rest, increased during eating. 5. When arterial blood pressure was elevated by noradrenaline (2-5 micrograms/kg I.V.), renal sympathetic nerve activity during resting was almost completely inhibited and the increase in renal sympathetic nerve activity during eating was not induced. 6. We conclude that renal sympathetic nerve activity increases in association with eating behaviour but not as firmly with the food presentation, and that the increase in renal sympathetic nerve activity is initiated by descending input from the higher central nervous system rather than either by the viscero-autonomic reflex due to food intake or by the baroreflex due to a decrease in arterial blood pressure.