Most of physiologically induced examples of recruitment of brown adipose tissue (BAT) occur as a consequence of chronic sympathetic stimulation (norepinephrine release within the tissue). However, in some physiological contexts (e.g. prenatal and prehibernation recruitment), this pathway is contra-indicated. Thus, a non-sympathetically mediated mechanism of BAT recruitment must exist. Here we have tested whether a PPARg pathway could recruit BAT fully independently of sympathetic stimulation by continuously treating brown (pre)adipocytes in culture with the potent PPARg agonist rosiglitazone. We found that this drug accelerated differentiation of brown adipocytes in culture (judged by lipid accumulation and expression levels of general adipogenic and BAT-related genes). Brown adipocytes in which PPARg was constantly in an activated state expressed UCP1 maximally even in the absence of norepinephrine. In cultures chronically treated with rosiglitazone, the expression of PPARg was markedly increased but this could not explain the brown-fat promoting effects of rosiglitazone, as these effects were still evident in PPARg-null cells. Strikingly, the isoform profile of the PPARg proteins was markedly altered in the presence of rosiglitazone: the PPARg1 protein became barely detectable, while the amount of PPARg2 protein become much higher than in control cells; the PPARg2 protein consisted mostly of novel PPARg2 protein isoforms formed by post-translational modification, including phosphorylation (at a site different from Ser-112) and with predominant nuclear localization. Thus, ligand-bound and presumably transcriptionally active PPARg2 apparently appears in rosiglitazone-treated brown adipocytes as a novel isoform with potent effects on brown adipogenic genes such as UCP1.