Extracellular levels of endogenous norepinephrine (NE) within rat paraventricular hypothalamus (PVN) vary across the diurnal cycle, with a peak in NE level noted at the onset of the dark cycle, at which time feeding occurs in a burst. The present experiment further examined the relationship between food intake and extracellular levels of NE within the PVN and within sites located outside of the hypothalamus. Adult male rats were implanted with concentric microdialysis probes aimed at either the PVN or brain sites outside the PVN. Measures of food intake and of extracellular NE were collected every hour over a 24-h period. Rats with PVN probes exhibited two peaks in extracellular NE. The first peak in PVN NE occurred within 1 h before the onset of the dark phase (ZT11) and was significantly correlated (p < 0.02) with a marked burst of feeding during the first hour of the dark phase. In addition, a second NE peak occurred 8 h into the dark phase (ZT19) but was not accompanied by feeding. Rats bearing non-PVN probes did not exhibit alterations in extracellular NE yet did show a pattern of feeding similar to that noted in the PVN rats. These data support the hypothesis that the burst of feeding evident at the onset of dark is related, in part, to an increase in PVN extracellular NE.(ABSTRACT TRUNCATED AT 250 WORDS)