The response of the digestive tract after ingestion of milk in lactase insufficiencies was investigated by X-ray. In subjects with lactose intolerance after milk the gastric evacuation is retarded, while there is a considerable simultaneous increase in the transit time through the small intestine. There is also an increased secretion in the small intestine. In the colon, pneumatosis starts from the moment the contrast substance reaches the caecum. These changes are not found after administration of lactose-free milk, where the findings are similar as in controls.