Given that angiotensin II (AII) type 1 and type 2 receptors (Agtr1 and Agtr2) are expressed in the adipose tissue, AII may act directly on the adipose tissue. However, whether or not AII modulates directly adipose tissue growth and metabolism in vivo and, if so, whether it is mediated via Agtr1 is still a matter of debate. To understand the functional role of Agtr1 in adipose tissue growth and metabolism in vivo, we examined the metabolic phenotypes of mice lacking Agtr1a (Agtr1a^-/- mice) during a high-fat diet. The Agtr1a^-/- mice exhibited the attenuation of diet-induced body weight gain and adiposity, and insulin resistance relative to wild-type littermates (Agtr1a^+/+ mice). They also showed increased energy expenditure accompanied by sympathetic activation, as revealed by increased rectal temperature and oxygen consumption, increased expression of UCP-1 mRNA in the brown adipose tissue, and increased urinary catecholamine excretion. The heterozygous Agtr1a-deficient mice (Agtr1a^+/- mice) also exhibited the metabolic phenotypes similar to those of Agtr1a^-/- mice. Using mouse embryonic fibroblasts (MEFs) derived from Agtr1a^+/+ and Agtr1a^-/- mice, we found no significant difference between genotypes in the capability of differentiating into lipid-laden mature adipocytes. In primary cultures of mouse mature adipocytes, AII increased expression of mRNAs for some adipocytokines, which was abolished by pharmacologic blockade of Agtr1. This study demonstrates that Agtr1a^-/- mice exhibit the attenuation of diet-induced weight gain and adiposity through increased energy expenditure. The data also suggest that AII does not affect directly adipocyte differentiation but can modulate adipocytokine production via Agtr1.