A decreased influence of nitric oxide (NO) in the peripheral vasculature is associated with the pathophysiology of established hypertension, and some studies suggest that increased blood pressure positively correlates with decreased NO production. If so, then the increased arterial pressure in one-kidney, one-clip (1K1C) hypertensive rats should be associated with decreased circulating levels of nitrite/nitrate (NO2/NO3; stable metabolites of NO) and guanosine 3',5'-cyclic monophosphate (cGMP; mediator of NO action). We measured serum NO2/NO3 and cGMP levels in early hypertensive 1K1C (2 wk after clipping) and shamoperated one-kidney (1K) normotensive rats, treated orally with or without the NO-synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 2 wk). Compared with those in 1K rats, NO2/NO3 and cGMP levels were increased in 1K1C hypertensive rats but not in 1K1C rats treated with L-NAME. NO2/NO3 and cGMP levels in L-NAME-treated 1K and 1K1C rats were similar. Compared with that in 1K rats, systolic blood pressure (SBP) was increased in 1K1C rats and in L-NAME-treated 1K and 1K1C rats. The SBP increase in L-NAME-treated 1K1C rats was more rapid than in untreated 1K1C rats. In early hypertension, increases in SBP positively correlated with increases in serum NO2/NO3 and cGMP. After 2 wk of hypertension, circulating NO2/NO3 levels gradually declined and reached prehypertension levels by the fifth week of hypertension. These results provide evidence for increased NO synthesis in early hypertensive 1K1C rats, and this increased NO could be a compensatory mechanism to slow the development of hypertension in these animals.