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Background The haemodynamic effects of intravenous infusion of the non-selective nitric oxide synthase (NOS) l-omega monomethyl arginine (l-NMMA) have previously been characterized in humans. Its effect of reducing cardiac index (CI) is an important reason for the increase in mortality in patients with septic shock receiving l-NMMA in a pivotal outcome trial for this indication. The mechanism for the reduction in CI however, is uncertain. Methods In this study, we investigated the haemodynamic and arterial stiffness response to a bolus intravenous infusion of l-NMMA (3 mg kg−1 over 5 min) in 26 healthy human volunteers to clarify the likely cause of l-NMMA induced negative inotropic and chronotropic effects. Digital photoplethysmography (MicroMedical Pulse Trace) was used to derive two measures of arterial stiffness: stiffness index, a measure of large arterial stiffness, and reflection index (RI), a measure of small- to medium-sized arterial stiffness. Haemodynamic measurements of systolic blood pressure, diastolic blood pressure, heart rate, systemic vascular resistance index (SVRI), stroke index and CI were made using a bioimpedance monitor (BioZ Cardiodynamics). Results We found that changes in CI during l-NMMA are closely related to changes in RI and SVRI. Conclusion The negative inotropic effect of l-NMMA may be a result of an increase in coronary vascular resistance and a resultant decrease in myocardial perfusion. The reduction in CI may also result from a direct reduction of the normal positive inotropic effect of NO by l-NMMA which is closely correlated with its effects on SVRI.