Nitric oxide synthase, the mammalian enzyme catalyzing the oxidation of L-arginine to L-citrulline and nitric oxide, is present in three isoforms that have distinct physiological roles. Overstimulation or overexpression of individual nitric oxide synthase isoforms plays a role in a wide range of disorders including septic shock, arthritis, diabetes, ischemia-reperfusion injury, pain and various neurodegenerative diseases. Animal studies and early clinical trials suggest that nitric oxide synthase inhibitors could be therapeutic in many of these disorders, but preservation of physiologically important nitric oxide synthase functions might require use of isoform-selective inhibitors. Within the past few years both amino acid and nonamino acid nitric oxide synthase inhibitors with pharmacologically useful isoform selectivity have been reported. Selectivity has been achieved on the basis of initial binding affinity and, for mechanism-based inactivators, on the basis of isoform-dependent catalytic activation; particularly interesting are N5-(1-imino-3-butenyl)-L-ornithine, ARL 17477, 1400W and S-(2-aminoethyl)isothiourea.