Monolayers of feline parotid acinar cells were infected with feline rhinotracheitis herpes virus. Membrane potentials and input resistances were measured before and during infection. Uninfected acinar cells had an average membrane potential of -51 mV and resistances from 6 to 120 mega-ohms; cells infected for more than 18 h had an average membrane potential of -19 mV and resistances from 30 to 100 mega-ohms. The time course of the decrease in membrane potential was studied and was found to commence about ten to 14 h after infection.