Hunger in pregnant sows maintained on commercial restricted rations was measured using an operant conditioning technique. Six sows were trained to lift a lever for a 2·7 g pellet reinforcement on a ratio of 10 lifts per reinforcement. After 1 h of stable responding (no pause greater than 5 min) the ratio was increased to 20 lifts. The point at which a sow stopped responding to this increasing ratio was used as a measure of strength of feeding motivation. Extinction was defined as occurring if the sow did not respond for a 2-h period. The six sows reached extinction at ratios of 70, 130, 210, 210, 230, and 430, and had stable intakes prior to extinction of 15, 22, 17, 10, 11, and 3 reinforcements per hour. Estimates of net energy deficits during responding prior to extinction were 271, 45, 271, 75, 242, and 441 kJlh. These results indicate that sows on restricted rations are still extremely hungry, to the extent that they are prepared to sustain an energy deficit to gain additional food.