Insulin resistance in peripheral tissues is a hallmark of human obesity and non-insulin-dependent diabetes mellitus (NIDDM). Thiazolidinediones are oral insulin-sensitizing agents that enhance glucose utilization without stimulating insulin release. Peroxisome proliferator-activated receptors (PPARs) are ligand-activated nuclear receptors, and PPARγ—selectively expressed in adipocytes and critical for adipocyte differentiation—was previously identified to have thiazolidinedione 6 as its first high-affinity ligand. This study investigates the structure-activity relationship (SAR) between the in vitro PPARγ agonist activity of compounds and their in vivo antihyperglycemic activity in genetically obese and diabetic mice. A panel of compounds (1b-11) with reported antihyperglycemic activity in rodent models were assessed for PPAR activation using GAL4-PPAR chimera transient transfection assays in CV-1 cells and for direct binding to PPARγ via [³H]-6 competition assays. All compounds activated PPARγ, with their in vitro EC50 values (potency for PPARγ activation) showing a strong positive correlation (r²=0.92, P<0.0005) with their in vivo minimum effective doses (MED) for antihyperglycemic activity in ob/ob mice. Notably, thiazolidinediones 3-11 were selective PPARγ agonists, while acids 1b and 2 showed weaker activity on both PPARα and PPARγ. No compounds activated PPARδ. The SAR of PPARγ agonist activity in vitro accurately predicts the in vivo antihyperglycemic efficacy of thiazolidinediones. This work establishes PPARγ as the molecular target for thiazolidinediones' antidiabetic effects and demonstrates that radioligand binding and transactivation assays can accelerate the development of more potent and selective PPARγ-targeted antidiabetic agents.