To assess the mechanisms of insulin resistance in the ruminant, severe and adult-onset obesity was produced in Dorset ewes by overfeeding a high-energy ration over a 1- to 2-yr period. Body weights increased to 100 kg compared with 50 kg in lean control sheep; significant hyperinsulinemia (40 +/- 4 vs 10 +/- 1 microU/ml) also developed as did a moderate hyperglycemia (62 +/- 2 vs. 52 +/- 1 mg/100 ml). Tissue sensitivity and responsiveness to insulin were then determined in both obese and lean sheep by the euglycemic glucose-clamp technique. Insulin was infused at eight different rates from 0.2 to 50 mU.kg-1.min-1 and [6-3H]-glucose was infused for measurement of glucose kinetics. The mean dose-response curves for glucose utilization and clearance rates were displaced to the right in obese compared with lean sheep. As a result, the half-maximally effective insulin concentrations usually were elevated two- to fourfold, indicating decreased insulin sensitivity in obese sheep, and this is consistent with decreased insulin receptors in peripheral tissues. On the basis of fat-free body weight, the maximal glucose responses, however, were not significantly different between obese and lean sheep, indicating that postreceptor defects do not exist in muscle tissue. Furthermore, lean ruminants are more resistant to insulin than are humans, but this resistance seems only because of the sheep's decreased responsiveness to insulin and thus only because of postreceptor insulin effects in peripheral tissues.