When a diet deficient in Val with excessive Leu is offered to pigs, feed intake is reduced. The physiological mechanisms underlying the rejection of branched-chain AA (BCAA) unbalance are unknown. A meal pattern analysis was performed to determine how pigs respond to the ingestion of a diet unbalanced in BCAA to identify the mechanisms responsible for the aversive response of such a diet. Eight female pigs (initial BW of 30.7 +/- 1.7 kg at 9 wk of age) received alternatively a diet either unbalanced in BCAA supply (i.e., a diet deficient in Val with an excess supply of Leu) or a control diet (i.e., a diet with a sufficient supply of Val with an excess supply of Leu) during 4 periods of 5 consecutive days. Feed consumption was monitored continuously to determine feeding behavior. Average daily feed intake of pigs receiving the diet unbalanced in BCAA was l3% less than that in pigs offered the control diet (P < 0.01). Offering the unbalanced diet had no effect on meal size but reduced the number of meals from 9.9 to 8.5 per day (P < 0.01). There was no difference between diets in intermeal interval but a slight trend for an increased meal duration from 14.5 to 16.3 min in pigs receiving the unbalanced diet (P = 0.13). The rejection of feed appeared within the first 2 d after offering the unbalanced diet. The results of this experiment indicate that the BCAA unbalanced diet had no effect on satiety and satiation. After initiation of a meal, pigs ate a normal-sized meal and were hungry, but the increase in meal duration and a decrease in the number of meals inferred a negative alliesthesia (less pleasant) response to the BCAA unbalanced diet. On the basis of meal patterns, the rejection of a diet unbalanced in BCAA may not involve short-term mechanisms related to oropharyngeal and gastrointestinal receptors but may be triggered by postabsorptive signals.