Cerebral hypertension (HiBP) increases cGMP levels in the newborn, indicating an increase in Nitric Oxide (NO). This effect was not inhibited but in fact increased by L-nitroarginine (LNA, a nitric oxide synthase (NOS) inhibitor). We determined the effect of LNA on NOS expression in neonatal cerebral microvessels and brain during normotension and induced hypertension. Four groups of sedated newborn piglets (n=4), (3-5 days old) were studied: two were normotensive (NMBP) groups (MABP range 65-85 mmHg), 1) control group given normal saline 0.5ml into cerebral ventricles (ICV), 2) LNA group given 3mg/kg of LNA (ICV), and two were hypertensive groups (MABP range 100-125 mmHg), 3) Control (HiBP) group given saline 0.5ml (ICV) and 4) LNA (HiBP) group given 3 mg/kg of LNA (ICV). Hypertension was induced by inflating a balloon tip catheter in the descending aorta, at 30 min and 2 hr post treatment. The brains were perfused after the 2 hr time point and the microvessels (CMVL) and brain tissue harvested for determination of protein and mRNA levels of eNOS and bNOS. The magnitude of change was determined by Western, Northern Blot and semiquantitative rtPCR analysis. Data show that LNA(NMBP) increased eNOS in both protein and mRNA levels by 2-3 fold in the brain and CMVL. The LNA (HiBP group) also increased with an additive effect on eNOS protein (2-3 fold) and mRNA (4-5 fold) levels. The change in eNOS expression was not different between LNA(NMBP) and LNA(HiBP) groups (p=0.17, r=0.6360) for CMVL; but was significantly different in brain tissue (p=0.035, r=0.843). Similarly, bNOS mRNA was also induced by LNA. bNOS mRNA levels were 5-7 fold greater in both LNA(NMBP) and LNA(HiBP) treatment groups in brain and CMVL compared to controls. Significant changes in protein bNOS levels was observed in the CMVL of the LNA(NMBP) treatment group (p=.0072, r=0.966). Our data indicate that treatment of normotensive and hypertensive animals with LNA increases the production of bNOS and eNOS after 2 hr in brain and CMVL. Thus, increased cGMP levels during cerebral hyperemia is caused by significantly induced expression of two major NO synthases by LNA (ICV). This suggests that LNA induced synthesis of eNOS and bNOS has an additive effect in the presence of cerebral hyperemia.