We hypothesized that levodopa with carbidopa, a common therapy for patients with Parkinson's disease, might contribute to the high prevalence of insulin resistance reported in patients with Parkinson's disease. We examined the effects of levodopa/carbidopa on glycogen concentration, glycogen synthase activity, and insulin-stimulated glucose transport in skeletal muscle, the predominant insulin-responsive tissue. In isolated muscle, levodopa/carbidopa completely prevented insulin-stimulated glycogen accumulation and glucose transport. The levodopa/carbidopa effects were blocked by propranolol, a beta-adrenergic antagonist. Levodopa/carbidopa also inhibited the insulin-stimulated increase in glycogen synthase activity, while propranolol attenuated this effect. Insulin-stimulated tyrosine phosphorylation of IRS-1 was reduced by levodopa/carbidopa, though Akt phosphorylation was unaffected by levodopa/carbidopa. A single in vivo dose of levodopa/carbidopa increased skeletal muscle cAMP concentrations, diminished glycogen synthase activity, and reduced tyrosine phosphorylation of IRS-1. A separate set of rats was treated intragastrically twice daily for 4 wks with levodopa/carbidopa. After 4 weeks of treatment, oral glucose tolerance was reduced in rats treated with drugs compared to control animals. Muscles from drug-treated rats contained at least 15% less glycogen and ~50% lower glycogen synthase activity compared to muscles from control rats. The data demonstrate beta-adrenergic dependent inhibition of insulin action by levodopa/carbidopa and suggest that unrecognized insulin resistance may exist in chronically treated patients with Parkinson's disease.