Insulin detemir (DET) is a basal insulin analog that, in contrast to other long-acting forms of insulin, has significant weight-gain sparing effects in diabetic patients. We hypothesized that this effect of DET may be due to its enhanced catabolic action in the CNS. We investigated the long-term effects of single third ventricular (3V) microinjections of equimolar doses of DET and regular insulin in normal male rats on feeding, body weight, energy expenditure (EE) and respiratory quotient (RQ). Also in acute testing, we assessed the ability of lower doses of DET to alter feeding, EE and RQ when microinjected directly into the paraventricular nucleus (PVN). The anabolic peptide ghrelin served as a positive control in acute testing. 3V administration of both DET (0.5-2.0 mU) and regular insulin (2.0-8.0 mU) significantly reduced feeding and body weight over 48 and 120 hrs, respectively, with DET yielding greater inhibitory effects. DET also stimulated greater elevations of EE and reductions of RQ over 72 and 48 hrs postinjection, respectively. In acute (4-hr) testing, microinjections of DET (0.5 mU) into the PVN reduced feeding, increased EE and reduced RQ, while ghrelin (100 pmol) had the opposite effects. When administered sequentially into the PVN, DET (0.25 and 0.5 mU) reversed ghrelin-induced feeding, EE and RQ effects. These data support the notion that the weight-sparing effect of DET is at least in part based on its central catabolic action, and that enhanced EE and reduced RQ may participate in this effect.