Red blood cells can be loaded with low molecular weight nitrosylating agents, such as S-nitrosothiols, to act as a delivery system for NO.sup.+ groups to tissues. Loaded red blood cells can be used in methods of therapy for conditions which are characterized by abnormal O.sub.2 metabolism of tissues, oxygen-related toxicity, abnormal vascular tone, abnormal red blood cell adhesion, or abnormal O.sub.2 delivery by red blood cells. Such treatment of red blood cells can be extended to in vivo therapies, with the object to achieve an increase in the ratio of red blood cell S-nitrosothiol to hemoglobin.