The behavioral development of suckling and intake control was studied in rat pups. Pups were observed at the initiation, during the course, and at the termination of suckling from their anesthetized mothers. Diet was delivered intraorally through a fine tongue cannula which enabled control of timing and volume. The control of diet intake and the behavior at termination of suckling showed correlated changes from 5-20 days or age. When deprived of suckling (and food and water) for 8 h, 5 and 10 day old pups concumed large volumes of diet (10% of body weight or greater) and terminated suckling only in the presence of extreme gastrointestinal filling. These young pups were immediately lethargic and slept after intake termination. Five day old pups persisted in reattaching to the nipple when manually stimulated; 10 day old pups eventually refused to reattach. In contrast, 20 day old pups consumed more moderate volumes of diet (5% of body weight). These pups also remained awake for a period after feeding and engaged in the exploratory and grooming activities characteristic of adult rats at the termination of feeding. These observations demonstrate major changes in suckling behavior during development. Intake control processes may shift from indirect to direct and become more effective and specifically food intake related in older pups.