The effect of Rolipram, a selective inhibitor of the cyclic AMP specific phosphodiesterase (PDE IV) was evaluated in the rat collagen type II (RCII)-induced arthritis model in the DA rat. Rolipram was given either shortly before expected onset of disease (days 10-14) or shortly after the onset of clinically evident arthritis (days 15-19 after immunization). Administration at days 10-14 delayed the onset of arthritis for approximately 5 days, but the severity of arthritis was thereafter comparable to that seen in a non-treated control group. Rolipram treatment of animals with manifest arthritis inhibited further arthritis development and also tended to diminish its severity at a phase of disease where non-treated control animals showed a rapidly progressing disease development. Serum levels of antibodies to RCII were in all experiments similar between Rolipram-treated and control animals. An in situ hybridization method for determining cytokine mRNA synthesis in regional lymph nodes, after administration of Rolipram (at days 2-7), demonstrated a strong inhibitory effect on tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) mRNA expression, whereas no effects were seen on IL-2 mRNA synthesis after in vivo challenge with native RCII emulsified in Freund's incomplete adjuvant. The results thus demonstrate strong preventive as well as therapeutic effects of Rolipram in a model for arthritis that is very similar to human rheumatoid arthritis with respect to cytokine regulation, and suggest that Rolipram has its major effects in the effector stage of the arthritogenic immune response.