The short-term distribution kinetics of 18F, with and without added carrier, were studied in 12 soft tissues and femur following intravenous administration in rats. The animals were killed in groups of four at 5, 10, 15, 20, 30, and 60 min after the dose. A tissue was judged to be kinetically homogeneous with plasma if an early and constant tissue water-to-plasma (T/P) 18F concentration ratio was established. In the carrier-free study, liver, heart, skin, fat, and kidney met this criterion. In the presence of carrier, four other tissues were added to this group. Brain, resting skeletal muscle, spleen, and femur did not achieve constant T/P values. The addition of carrier resulted in significantly lower T/P ratios in nine tissues. This was especially marked in femur. It is concluded that 1) none of the soft tissues studied strongly binds 18F; 2) most of these tissues are kinetically homogeneous with plasma; and 3) the presence of carrier fluoride can significantly influence the kinetic behavior and distribution of 18F. Finally, based on the current and previously reported 18F T/P ratios, and on reported intracellular-extracellular pH gradients, it is hypothesized that fluoride distribution in several soft tissues is determined by the diffusion equilibrium of HF, that is, by the magnitude of the transmembrane pH gradient.