Intracellular recording techniques havebeen used to measure transmembrane potentials (TMP), membrane resistance, and membrane capacitance from individual cells in the rat adenohypophysis in vivo. Transmembrane potential and membrane resistancewere monitored following intracarotid injection of hypothalamic stalk-median-eminence (HSME) extract. The injection of the equivalent of 1 rat HSME resulted in a transient alteration in TMP in only a small proportion of the cells, but produced a consistent and reproducible increase in membrane resistance in the majority (77%) of cells. The results suggest that cells in the adenohypophysis may be coupled with low resistance intercellular bridges. Releasing factors may function to uncouple cells.