Cells containing arginine vasopressin (AVP)- and oxytocin (OXT)-like substances were immunohistochemically visualized in the cerebral, subesophageal, and ventral nerve cord ganglia of the earthworm Pheretima hilgendorfi. Whether these anti-AVP- and anti-OXT-reactive cells are identical with classical aldehyde fuchsin (AF)-positive neurosecretory cells was tested in serial sections. In all ganglia, groups of scattered neuronal cell bodies and axons strongly reactive to AVP and OXT antisera were observed, but AF-positive cells consisting of type a (dark blue) and type b (purple) cells were predominantly present in the cerebral and subesophageal ganglia. In the cerebral and subesophageal ganglia anti-AVP- and anti-OXT-reactive cells were generally larger than AF-positive cells. Some AF-positive cells were reactive either to anti-AVP or anti-OXT serum, but some failed to react to either serum. Anti-AVP- and anti-OXT-reactive cells were not immunoreactive to OXT and AVP antisera, respectively. Electron microscopic observations showed that the granules of type a cells were larger and less electron dense than those of type b cells and anti-AVP-reactive cells. The present cytological observations clearly showed that AVP- and OXT-like substances were widely present in the ganglionic cells of the earthworm.