Gap junctions of normal and neoplastic astrocytes were studied by the use of freeze-fracture or-etching technique. The gap junctions between normal astrocytes generally revealed a structure composed of small particles on fracture face A and associated pits on fracture face B. The particles and pits had presumably a complementary correlation: however, the pits were usually more regularly oriented—often in hexagonal array—than the particles. In the astrocytoma, the gap junctions were abundant and the pits were occasionally in tetragonal array. In the glioblastoma multiforme, the number of gap junctions was markedly decreased and the space between the particles sometimes increased, indicating a loose arrangement of the particles. The gap junctions in normal and neoplastic astrocytes demonstrated discontinuous macular plaques and did not exhibit continuous zones around the astrocytes. There was no evidence of tight junction in the fracture replicas of astrocytes. The present study might suggest that the malignancy of neoplastic astrocytes is represented by the decrease of the gap junction or by the loose orientation of the particles in the gap junction.