Regrouping is a common management practice on commercial dairy farms. The objectives of this study were to examine the effects of regrouping on feeding behavior, social behavior, and milk production. Eleven mid-lactation Holstein cows were individually introduced into previously established social groups. Behavior and milk production were monitored from 3 d before until 3 d after regrouping. Cows were fed a total mixed ration ad libitum twice daily. Time-lapse video was used to quantify feeding and social behavior. Cows spent approximately 15 min less time eating in the first hour following regrouping compared with the 3 d before regrouping. Cows were displaced from the feeding area on average 10 times/d before regrouping, but this value increased to more than 25 times ( +/- 2.6) on the day of mixing and gradually declined on the days following. The number of lying bouts declined from 12.2 +/- 0.9 to 10.5 +/- 0.9 on the day of regrouping; lying time showed a similar tendency. Cows initiated, on average, 7.5 +/- 1.3 allogrooming events/d over the 3 d before regrouping but this declined to 1.3 +/- 1.3 events on the day of mixing and remained lower than baseline for the following 3 d. Milk production declined from 43.4 +/- 1.5 kg/d to 39.7 +/- 1.5 kg/d on the day of regrouping, but did not differ from premixing levels on subsequent days. This study shows that regrouping can disrupt behavior and production in the hours and days following regrouping and suggests the need for future research to identify management changes that reduce these effects.