The peroxisome proliferator activated receptor PPAR-γ has been identified as a nuclear receptor for thiazolidenediones, which are compounds with insulin-sensitizing properties in several tissues, including skeletal muscle. To determine whether this receptor is expressed and possibly involved in insulin action/resistance in skeletal muscle, PPAR-γ mRNA abundance and its regulation by insulin were quantified in muscle tissue and cultures from lean and obese nondiabetic and type II diabetic subjects using competitive reverse transcription–polymerase chain reaction (RT-PCR). In muscle biopsy specimens, PPAR-γ mRNA was elevated in obese nondiabetic and type II diabetic subjects (23.4 ± 4.2 and 28.0 ± 5.69 × 10³ copies/µg total RNA, respectively; both P < 0.05) compared with lean nondiabetic control subjects (9.4 ± 2.3 × 10³ copies/µg total RNA). Significant positive correlations were present among skeletal muscle PPAR-γ mRNA levels, BMI (r = 0.67, P < 0.01), and fasting insulin concentration (r = 0.76, P < 0.001). PPAR-γ mRNA levels were also elevated in muscle cultures from type II diabetic subjects compared with lean nondiabetic control subjects (330.1 ± 52.9 vs. 192.1 ± 27.0 × 10³ copies/µg total RNA, P < 0.05). Insulin stimulation of muscle tissue (by hyperinsulinemic- euglycemic clamp for 3–4 h) or muscle cultures (30 nmol/1 for 120 min) stimulated PPAR-γ mRNA expression up to fourfold (10.0 ± 2.7 to 41.3 ± 7.4 × 10³ copies/µg total RNA, P < 0.05, and 174.9 ± 56.9 to 268.2 ± 78.6 × 10³ copies/µg total RNA, P < 0.05, respectively). In summary, PPAR-γ mRNA expression in human skeletal muscle is acutely regulated by insulin and is increased in both obese nondiabetic and type II diabetic subjects in direct relation to BMI and fasting insulinemia. We conclude that abnormalities of PPAR-γ may be involved in skeletal muscle insulin resistance of obesity and type II diabetes.