Recent studies with rat pups suggest that suckling and sleeping are coordinated through milk-related events in the gut. Our experiments revealed that suckling rats respond to milk in the upper gastrointestinal tract by displaying more paradoxical sleep (PS) as the volume increases to 4% of the pup's body weight. Conversely, gastric loads larger than 4% reduced PS as a function of the volume. We also discovered that filling the stomach with warm non-nutritive paraffin is as effective as an equivalent volume of warm milk for enhancing PS. Although the temperature of the gut load did not appear to play a major role in the amount of PS displayed, increasing ambient temperature from 22 degrees C to 32 degrees C increased PS significantly. Moreover, a gut load of milk (4% body weight) was more effective than the same volume of water or no load for enhancing PS. Gut loads that stay in the stomach and warm ambient temperature appear to work in an additive manner to enhance PS. The electrophysiological data together with the stomach volume data and behavioral observations of nipple attachment revealed that milk-related stimuli along the gastrointestinal tract, especially gastric distension, alter sleep patterns in predictable ways that permit us to distinguish postingestive satiety from a deprivation state and nimiety in suckling rats.