We have studied the in vivo and in vitro effects of Topiramate (TPM) in female Zucker Diabetic Fatty (ZDF) rats. After weight matching, drug treatment had a marked effect to lower fasting glucose levels of relatively normoglycemic animals as well as during an oral glucose tolerance test (OGTT). The glucose clamp studies revealed a ~30% increased glucose disposal, increased HGO suppression from ~30 to 60%, and an increased FFA suppression from 40 to 75%. Therefore, TPM treatment led to enhanced insulin sensitivity at the level of tissue glucose disposal (increased ISGDR), liver (increased inhibition of HGO), and adipose tissue (enhanced suppression of lipolysis). When soleus muscle strips of Control or TPM treated ZDF rats were studied ex vivo, insulin stimulated glucose transport was not enhanced in the drug treated animals. In contrast, when isolated adipocytes were studied ex vivo, a marked increase (+55%) in insulin stimulated glucose transport was observed. In vitro treatment of muscle strips and rat adipocytes showed no effect on glucose transport in muscle with a 40% increase in insulin stimulated adipocyte glucose transport. In conclusion: (1) TPM treatment leads to a decrease in plasma glucose and increased in vivo insulin sensitivity (2) insulin sensitization was observed in adipocytes, but not muscle when tissues were studied ex vivo or in vitro and (3) TPM directly enhances insulin action in insulin resistant adipose cells in vitro. Thus, the in vivo effects of TPM treatment appear to be exerted through adipose tissue.