The decrease of plasma insulin levels after physical training is explained mainly by a decreased insulin secretion, but also to a significant degree by an increased peripheral insulin metabolic clearance rate in several peripheral tissues and increased insulin uptake in the liver. The peripheral tissues demonstrate an increased insulin sensitivity, but this seems to be due to factors after the insulin receptor level. An increased turnover of insulin at the receptor level might be postulated. The explanation for the decrease of insulin secretion has been studied extensively. A decreased cortisol secretion might be a primary factor in physical training, followed by lower insulin levels. Autonomic nervous system adaptations could also be involved in the form of an increased alpha-adrenergic activity. Another alternative is that the effect is mediated via an influence of physical training on a gut "incretin" factor, facilitating insulin secretion, Finally, it might be considered that the increased insulin sensitivity of muscle following physical training is regulating insulin secretion via feedback mechanisms to the beta cell. Recent demonstrations of inhibited glucose uptake in the liver after exercise facilitates an explanation along these lines.