A variety of experiments involving direct measurement of membrane potentials with microelectrodes have led to the notion that the generalized chemiosmotic hypothesis (4) can account for the major transport processes occurring in the plasma membrane of Neurospora; and that fungal plasma membranes, despite their apparent lack of redox elements, are useful models for many transport events which take place in energyconserving membranes. The diagram in Fig. 1 represents the current working hypothesis for transport by Neurospora. The measured resting membrane potential lies in the range -160 to -240 mV (cell interior negative) and is sustained largely by ejection of protons coupled to the splitting of ATP (8).' The membrane potential then serves as the main energy distributor for transport, and-together with a small inward chemical gradient for protons-drives the uptake of a variety of different substances. Direct evidence exists that glucose and its analogues are handled in this manner (9), and indirect evidence that amino acids, phosphate, and potassium are as well.2 From the measured membrane resistance (ca. l0,000Q cm2 under standard conditions; ref. 7) and potential, the ion flux driven by the proton pump can be estimated at 200 pmol/cm2 s. If the density of transport sites on the membrane were roughly the same as that of F,-ATPase sites on the mitochondrial inner membrane (3,000/im2, ref. 1), the flux would yield a turnover rate of 200/site * s, for a two-charge process. Major evidence in support of Fig. 1 comes from a survey of the speed and extent of depolarizations caused by certain ions, metabolic inhibitors, and antibiotics. Hydrogen ions, almost alone among the common inorganic cations and anions, have a steep depolarizing effect: 30-40 mV for each unit decrease of pH,, at least in the range pH 5 to pH 3 (6). Cyanide, at concentrations just sufficient to block cytochrome oxidase,