The involvement of neuron-derived NO in the process of orienting and scanning times (non-selective attention: NSA) towards environmental stimuli has been investigated in the Naples High-Excitability rat (NHE), a putative animal model of Hyperactivity and Attention Deficit (ADHD). To this aim, orienting and scanning times have been monitored by the frequency and duration of rearing episodes, respectively. Adult male NHE rats were tested in a novelty situation (Làt-maze) for 30 min following single or repeated injections of the non competitive inhibitor 7-Nitroindazole (7-NINA) of the neuronal isoform of the enzyme nitric oxide synthase (n-NOS). In the acute experiments, rats received a single injection of 7-NINA (1 mg/kg) intraperitonealy in a saline vehicle (exp. 1, fast release) or subcutaneously in a lipid carrier, dimethyl sulfoxide (DMSO; exp. 2, slow release) or the vehicles alone as controls 30 min before testing. In the repeated injection experiments, rats received a subcutaneus injection of 1 mg/kg in DMSO or DMSO alone daily for 14 days, and tested 24 h after the last injection (exp. 3, slow release). The results showed a significant differential effect of the drug that was dependent on the release rate, i.p. saline-diluted 7-NINA increased the duration of individual rearing episodes whereas, both single and repeated subcutaneous DMSO-carried 7-NINA exerted an opposite effect. Thus, selective inhibition of n-NOS by an allosteric inhibitor that increases arginine availability without displacing the inhibitor from n-NOS, strengthens the hypothesized role of NO in NSA. These findings may shed light on the mechanism of action of drug treatment of and be useful in the treatment of ADHD in children.