Lactating guinea pigs (Cavia porcellus) and their litters were observed by videophotography across the light/dark cycle at 1, 11, 21, and 31 days postpartum. The highest level of behavioral activity was seen in the dark, particularly in the hour after light offset. This circadian pattern was evident from Day 1 in mothers and from Day 11 in pups. Contact between mothers and pups was inversely related to activity, occurring more frequently during light. Maternal grooming of pups occurred on Day 1 and then declined; self-grooming by pups increased across days. Intake of solid food and water by pups occurred on Day 1 and increased thereafter. A nearly complete transition from nursing to independent ingestion was observed between 21–31 days of age. Overall, we document several ontogenetic changes in young guinea pigs and demonstrate that under laboratory conditions mother and infant guinea pigs exhibit a nocturnal activity pattern.