A rapid feedback signal is not always necessary for termination of a drinking bout

When a pig is deprived of drinking water a deficit of body water develops t hat is corrected when the pig drinks to satiation. If food is available dur ing the deprivation, the stimulus to drinking is plasma hyperosmolality. Be cause of the delay in correction of plasma hyperosmolality as ingested wate r is slowly absorbed, it has been thought that a rapid inhibitory signal fr om the digestive tract is necessary to prevent overdrinking. This concept w as tested by measuring changes in plasma osmolality before and during drink ing after such deprivation and also after infusion of hypertonic saline. As drinking began, there was a rapid fall of plasma osmolality to levels insu fficient to drive drinking by the time drinking ended. This fall of plasma hyperosmolality to subthreshold levels while the pig is drinking seems to m ake a rapid inhibitory control signal from the digestive tract unnecessary to terminate the drinking bout under these conditions.