Insulin binding to liver microsomes from lean (Yorkshire) and obese (Ossabaw) swine was measured at four different times during growth to market weight. Over the physiological range of insulin, binding decreased during growth to market weight in Yorkshire swine. This was not observed for Ossabaw swine. When comparisons of insulin binding were made between lean and obese swine of a similar age, microsomes from lean swine bound more insulin over the physiological range of insulin than microsomes of obese swine. This difference was also observed when comparisons were made at similar live weights. The dissociation constant (Kd) for the high affinity receptor population increased with growth in both breeds indicating that binding affinity was decreasing. Over the physiological range of insulin, binding affinity was lower for liver microsomes of obese swine vs liver microsomes from lean swine when comparisons were made at a similar age. These results suggest that the liver of obese swine is moderately insensitive to insulin and that binding of insulin to liver from lean swine declines during growth to market weight.