It has been postulated that a cold-induced decrease in hypothalamic neuropeptide Y (NPY) might disinhibit the sympathetic outflow that activates brown adipose tissue (BAT) thermogenesis. The present work has assessed the interscapular BAT NPY and norepinephrine (NE) concentrations 7 days after surgical excision of the sympathetic nerves (Sx) of each BAT lobe in rats exposed to 4 degrees C or 24 degrees C during 24 h. In addition, the effects of NPY treatment on BAT oxygen (O2) consumption in normal and T3-treated hypothyroid rats was determined. Sprague-Dawley rats weighing 220-240 g were used. BAT was removed and homogenized in 0.1 M HCl. BAT NPY content at 24 degrees C was 109 +/- 19 pg/mg protein. Sx decreased this value significantly (P < 0.001) and cold increased it by two-fold (P < 0.001). Neither cold nor Sx altered hypothalamic NPY. BAT O2 consumption was depressed by NPY treatment (P < 0.001) and increased by NE (P < 0.001). In euthyroid Sx rats, NPY decreased O2 consumption (P < 0.001). NPY failed to alter O2 consumption in hypothyroid rats and sharply decreased it in T3-treated rats. The results show that NPY blocked the effects of T3 on BAT O2 consumption, and that cold-induced activation of the thermogenic process did not produce measurable changes in hypothalamic NPY.