The present study sought to evaluate concomitant alterations of behavioral and sleep patterns of arthritic rats. Rats were implanted with electrodes for polysomnographic recordings and submitted to the model of arthritis by a subcutaneous (s.c.) administration of Freund adjuvant in the posterior right paw and saline in the posterior left paw. The SHAM group was injected with saline in both paws, whereas the control group (CTL) was not submitted to any manipulation. Behavioral tests were carried out twice before induction of arthritis, on the second day of arthritis, and once a week afterwards until the eighth week. Body weight, colonic temperature, and measurements of the injured paw were carried out on the same days. Arthritic rats presented a reduction of total sleep time, increased latency to synchronized sleep, augmented number of episodes of synchronized sleep, reduction of sleep efficiency, more stage shifts, and increased total alert time. Moreover, these animals presented a lower pain threshold than control and SHAM animals. This reduction was observed on the second day of arthritis and remained so reduced until the end of the study. The data appear to indicate a relationship between altered sleep pattern and increased pain sensitivity in arthritic rats.