Abstract Somatodendritic release of dopamine (DA) was studied by microdialysis. The basal release of endogenous extracellular DA in the rat's substantia nigra was 1.0 ± 0.14 fmol/min (mean ± S.E.M.; n = 6). Compared with the basal extracellular level of DA found in the striatum (12.9 ± 0.94; n = 12), the former value was about 13 times smaller than the latter value. The addition to the Ringer solution of the fast sodium channel inhibitor, tetrodotoxin (TTX), at a concentration of 1 μM, produced the total disappearance of DA in the substantia nigra. When TTX was removed from the Ringer solution, the DA extracellular level came back slowly to the control value. This result suggests that the somatodendritic basal endogenous DA release measured by microdialysis is dependent on the sodium channel conductance.