The first step in the metabolic conversion of a nitroheterocyclic drug is the reduction of the vitro group to give the corresponding nitro radical anion. Several reductases and low-molecular weight bio-molecules are known to reduce nitro compounds. We have shown in the present work that the nitroheterocyclic drugs can be reduced by intact rat hepatocytes, and the nitro radicals were formed inside the cell. By using line-broadening paramagnetic reagents, we have shown that the nitro radicals formed inside the cell "freely" cross the plasma membrane, and the radical detected by ESR is mainly due to the radical present outside the cell. This approach of using paramagnetic reagents to distinguish the nitro radical anion present inside the cell from those outside the cell can also be used to study radical cations. Hence, we studied the formation of bipyridylium radical cations by hepatocytes. Our results for these radicals are the same as that for nitro radical; namely, bipyridylium radicals are formed inside the cell, but cross the plasma membrane. The radicals detected by ESR are mostly present outside the cell. The radicals present inside the cell were detected when the ESR signal due to the radicals present outside was completely broadened. Monoglutathionyl 1,4-benzosemiquinone radical formed by rat hepatocytes in the presence of 1,4-benzoquinone is shown to be formed inside the cell, but again the radical detected by ESR is outside the cell. Glutathionyl conjugates are known to be exported by hepatocytes.