The properties of astroglial gap junction channels and the protein that constitutes the channels were characterized by immunocytochemical, molecular biological, and physiological techniques. Comparative immunocytochemical labeling utilizing different antibodies specific for liver connexin 32 and connexin 26 and antibodies to peptides corresponding to carboxy-terminal sequences of the heart gap junction protein (connexin 43) indicates that the predominant gap junction protein in astrocytes is connexin 43. The expression of this connexin in cultured astrocytes was also established by Western and Northern blot analyses. Cultured astrocytes expressed connexin 43 mRNA and did not contain detectable levels of the mRNAs encoding connexin 32 or connexin 26. Further, the cells contained the same primary connexin 43 translation product and the same phosphorylated forms as heart. Finally, electrophysiological recordings under voltage-clamp conditions revealed that astrocyte cell pairs were moderately well coupled, with an average junctional conductance of about 13 nS. Single-channel recordings indicated a unitary junctional conductance of about 50–60 pS, which is of the same order as that found in cultured rat cardiac myocytes, where the channel properties of connexin 43 were first described. Thus, physiological properties of gap junction channels appear to be determined by the connexin expressed, independent of the tissue type.