Ion transport in the presence of pore formers, carriers, and hydrophobic ions, has been studied at hydrostatic pressures to 100 MPa. The onset of voltage-dependent conductance produced by the pore forming antibiotic, alamethicin, following application of a suprathreshold voltage step, is markedly slowed and follows a distinctly sigmoidal time course at elevated pressure. This observation has led us to postulate the existence of a voltage-dependent but nonconducting precursor state through which aggregates of alamethicin must pass before a conducting state is reached. The lifetimes of individual states of the open alamethicin pore are also lengthened by pressure, their decay being characterized by positive activation volumes in the range 70-80 A ~. The steady state conductance due to transport of K รท ions by the ion-selective carriers, valinomycin and nonactin, decreases with increasing pressure, indicating positive activation volumes of 45 A ~ and 74 A3, respectively, for the rate limiting step in transport by these ionophores. The activation volumes reported above are comparable to those inferred from measurements of the pressure dependence of the viscosity of n-alkanes, suggesting that membrane fluidity is pertinent to these observations. A marked contrast is provided by the hydrophobic anions, dipicrylaminate and tetraphenylborate. The parameters characterizing transient conductance by these species exhibit no measurable variation with pressure over the range available with present apparatus.