Eight litters were studied from birth to 37 days of age. The effects of weaning on the physiological responses of each piglet within the litter were examined, with focus being on the effects of relative weight and weaning age on piglet responses. Four litters were weaned at 21 days of age, while the other four litters were weaned at 35 days of age. On the day of weaning, the piglets were moved to a separate weaning room and placed in straw-bedded pens with a heated creep box, a water bowl, and a 3-space feeder. They were allowed ad libitum access to both feed and water. Data on weight gain, behaviour, and salivary cortisol levels (as an indicator of stress) were collected throughout the study. In general, during the pre-weaning period, heavier pigs were more likely to occupy the sow’s front teats during suckling, they were more likely to make low-pitched vocalizations, they carried out less nosing behaviour, and they were more aggressive than lighter pigs. Similar results were found at weaning for the heavier pigs. Those pigs having heavier weaning weights had occupied the sow’s front teats most during suckling (pre-weaning). Heavier pigs made less high-pitched and more low-pitched vocalizations after weaning, and they continued to be more aggressive. As a result, heavier piglets experienced higher cortisol levels. During the first two days after weaning, there was an increase in high-pitched vocalizations for pigs weaned at 21 days. Frequency of low-pitched vocalizations increased for both treatments on the first day after weaning. Aggression, nosing and chewing behaviours, and cortisol levels were highest on the first day after weaning. Aggression and cortisol levels among pigs weaned earlier decreased faster than for pigs weaned later. Earlier weaning was more stressful than later weaning for both small and large pigs. However, the challenges they have to endure appear to differ. Larger pigs seemed to have more difficulty adapting to their nutritional demands. Smaller pigs may have more difficulty adapting to maternal separation, as they had received less milk during lactation.