The paper reviews evidence that adjuvant arthritis in the rat is associated with chronic pain and discusses the time course and measurement of this putative pain. The available evidence is consistent with the view that arthritic rats suffer pain, but it appears difficult to formally establish the occurrence of chronic pain in animals. The data suggest the pain to be severe during weeks 2 and 3 and to persist during weeks 4 and 5 after inoculation. The continuing inflammation of joints likely results in movement-induced acutely elicited pains that may persist till about the 8th week. The severe pain during weeks 2 and 3 may be associated with a depression of some drives, and the entire week 2-8 period is likely associated with varying levels of chronic stress. Neurochemical and neurophysiological studies indicate that adjuvant arthritis profoundly influences several of the neurotransmission and neuroendocrine functions of brain and spinal cord; among the affected systems are substance P-ergic, serotonergic and endorphinergic systems. Adjuvant arthritis in the rat constitutes the only laboratory animal model of chronic pain that has been validated to a significant extent. It is suggested that the model be examined further and that additional animal models of chronic pain be developed.