Singlet molecular oxygen, O-2(a(1)Delta(g)), can be created in a single cell from ground-state oxygen, O-2(X-3 Sigma(-)(g)), upon focused laser irradiation of an intracellular sensitizer. This cytotoxic species can subsequently be detected by its 1270 nm phosphorescence (a(1)Delta(g) -> X-3 Sigma(-)(g)) with subcellular spatial resolution. The singlet oxygen lifetime determines its diffusion distance and hence the intracellular volume element in which singlet-oxygen-initiated perturbation of the cell occurs. In this study, the time-resolved phosphorescence of singlet oxygen produced by the sensitizers chlorin (Chl) and 5,10,15,20-tetrakis(N-methyl-4-pyridyl)-21H,23H-porphine (TMPyP) was monitored. These molecules localize in different domains of a living cell. The data indicate that (i) the singlet oxygen lifetime and (it) the rate constant for singlet oxygen quenching by added NaN3 depend on whether Chl or TMPyP was the photosensitizer. These observations likely reflect differences in the chemical and physical constituency of a given subcellular domain (e.g., spatially dependent oxygen and NaN3 diffusion coefficients), thereby providing evidence that singlet oxygen responds to the inherent heterogeneity of a cell. Thus, despite a relatively long intracellular lifetime, singlet oxygen does not diffuse a great distance from its site of production. This is a consequence of an apparent intracellular viscosity that is comparatively large.