The mechanisms involved in brown adipose tissue (BAT) atrophy in obese rats were investigated. In urethan-anesthetized adult obese (fa/fa) and lean (Fa/?) rats, colonic temperature (Tc), interscapular BAT (IBAT) temperature (TIBAT), and the TIBAT-Tc gradient were measured after microknife cut in the prepontine region and during norepinephrine (NE) infusion. The knife cut tests the neural control of IBAT since it suppresses a tonic inhibition of thermogenesis revealing a sustained reflex stimulation of BAT. The NE infusion tests the metabolic capacity of BAT. In 22 degrees C-acclimated lean rats, the cut induced marked hyperthermia, TIBAT that rose faster than Tc, and a reversed temperature gradient. By contrast, in the obese rat, the cut had practically no effect, and NE infusion caused only slight increases in Tc and TIBAT. In cold-acclimated obese rats, an almost complete response to NE infusion and a partial response to knife cut were observed. After adrenalectomy, the responses were similar in lean and obese rats. An operational model to account for these results and for the deficit in diet-induced thermogenesis in fa/fa rats is proposed.