The role of nitric oxide (NO) in maintaining normal mechanical behavior of red blood cell (RBC) has been previously demonstrated. The effects of NO donor and NOS inhibitor on the mechanical properties of density fractionated RBC were tested in this study. A non-specific NOS inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME) at a concentration of 10(-3) M and sodium nitroprusside (SNP), a nitric oxide donor at a concentration of 10(-6) M was added to blood samples with hematocrit adjusted to 0.4 l/l and RBC deformability was measured by an ektacytometer in the density fractionated RBC after one hour incubation at 37 degrees C. There was no significant effect of the NO donor SNP on cellular deformability in the older (denser) RBC fraction in contrast with the younger (least dense) fraction. Alternatively, the sensitivity of cellular deformability to competitive NOS inhibition by L-NAME was greater in the older fraction. These findings suggest that older RBC are characterized by diminished internal NO synthesis and are also less sensitive to external NO indicating that the target mechanisms for NO may also be deteriorated.