Urinary c-AMP and c-GMP levels were measured in male and female F344/DuCrj rats during dietary treatment with 2% sodium o-phenylphenate (OPP-Na) for 136 days. At the end of the experiment, levels of the cyclic nucleotides in the plasma, kidney and liver, and of adenylate cyclase in the kidney and liver were also determined. Urinary c-AMP/creatinine in males decreased immediately after the start of OPP-Na treatment and recovered to the normal level within 1 wk. Such a biphasic change recurred intermittently until the end of the experiment. In the case of treated females, urinary c-AMP/creatinine also showed a decrease and recovery in 1 wk but no subsequent decrease was observed. As urinary c-GMP/creatinine in treated males was higher than that in control males, low c-AMP/c-GMP ratios persisted throughout the experiment. Although there was no significant difference of c-AMP or c-GMP levels in the liver or kidney between control and treated animals, c-AMP/c-GMP ratios in these tissues from treated rats decreased slightly. The c-AMP levels in plasma from treated males were significantly lower than those in the male controls, but no significant difference was observed between treated and control females. In the presence of activators (sodium fluoride, glucagon, epinephrine), adenylate cyclase activity in liver and kidney homogenates from treated rats was only about 70% of the control activity, except with the female kidney enzyme. This observed inhibition in treated rats suggests that the decreased excretion of urinary c-AMP in treated rats was not due to a decrease in renal clearance but to reduced production of c-AMP.