The influence of adjuvant arthritis in rats on main urinary metabolites (MUM) of prostaglandin (PG) F and E type was studied. 1) In our model rats, urinary excretion of PGE-MUM increased on day 11 prior to the appearance of secondary lesions, but that of PGF-MUM did not change significantly during the observed period (21 days). The excretion of PGE-MUM was not proportional to the increase in both hind paws volume. 2) Indomethacin, which diminished primary lesions but did not suppress secondary lesions, reduced the increase in urinary excretion of PGE-MUM. 3) Prednisolone and azathiopurine, which suppressed both primary and secondary lesions, increased the excretion of PGE-MUM over adjuvant control values on days 4 and 8. The facts described above suggest that the increase of endogenous PGE during days 4 to 8 may be important in the suppression of secondary lesions in adjuvant arthritis in rats.