Thirty-nine dogs were assigned to four treatment groups for weight loss. Breeds, sex and initial body condition were similar across groups. Treatments included 0, 25, 40 or 50% caloric restriction, based on calculated maintenance energy requirements for the dog's estimated ideal body-weight. True maintenance energy requirements and actual caloric restriction were determined retroactively. Dogs were videotaped simultaneously in the late afternoon, early morning, immediately before the morning feeding and immediately after the morning feeding during the 3 days prior to initiating caloric restriction, and on days 1-2, 8-9 and 15-16 of caloric restriction. Overall activity levels varied significantly, with the direction of change depending on size of dog, level of restriction, gender and time on diet. Small breed male dogs, which were on the most severe restriction based on actual caloric needs, showed an initial increase in activity prior to feeding, followed by decreasing activity as they continued on caloric restriction. Large breed male and female dogs, which were on less intense actual restriction, exhibited a slight increase in overall activity, deriving primarily from decreased sleep and increased sitting, standing and walking. There was a significant increase in object mouthing by dogs on caloric restriction. An early (day 1 ) increase in focused barking by dogs on greater caloric restriction is followed by a significant decrease in focused barking by dogs on greater caloric restriction. In summary, severe calorie restriction may be counter-productive by inducing decreases in activity.