OBJECTIVE: To compare the plasma amino acid (AA) concentrations obtained by the infusion of an AA solution (660 mOsm/L, pH 7) using the subcutaneous (SC) with that using the intravenous (i.v.) route in older patients. DESIGN: A prospective, randomized, cross-over study. SETTING: A hospital geriatric ward. PARTICIPANTS: Six patients with a mean age of 84 years. MEASUREMENTS: The infusion of the AA solution (IV or SC) lasted 6 hours. Blood was sampled at 0, 2, 4, 6, 8, 10, 14, 18, and 24 hours from the start of the infusion to determine plasma AA level by the phenyl-isothiocyanate method. RESULTS: Compared with baseline values, plasma AA concentrations increased to a significantly higher level from the second to the tenth hour and from the second to the fourteenth hour during i.v. and SC infusions, respectively. Plasma AA levels did not differ between the two routes. SC infusion was well tolerated. CONCLUSION: Assuming that nutritional sufficiency is the desired result of plasma AA infusion, we conclude the SC route is well tolerated and offers the possibility of nitrogen supplementation for older patients over short periods of time, when oral protein intake is transiently insufficient or impossible, as a way to limit, but not to treat, protein-energy malnutrition.