Abstract: Physiologically active N.sup.6 -(hydrazinoiminomethyl)lysine or pharmaceutically acceptable acid addition salt thereof is administered in a nitric oxide synthesis inhibiting amount to a subject in need of such inhibition (e.g., a subject with low blood pressure, e.g., due to sepsis or to therapeutic administration of cytokines, or needing immunosuppressive effect) or is added to a medium containing isolated organs, intact cells, cell homogenates or tissue homogenates in an amount sufficient to inhibit nitric oxide formation to elucide or control the biosynthesis, metabolism or physiological role of nitric oxide. CompThis invention was made at least in part with Government support under National Institutes of Health grant number DK 37116. The Government has certain rights in the invention.