The permeability transition process in rat liver mitochondria was studied by following the swelling consequent to external solute equilibration through the membrane pore. The kinetics of the transition was analyzed according to a model based on the assumptions that the transition rate follows a first-order process and that the solute diffusion rate strongly depends on the pore conformation. Three kinetic parameters, easily calculated from absorbance measurements during mitochondrial swelling, were used to determine whether changes of the swelling rate are due to changes of (i) the transition rate, (ii) the amount of permeabilized mitochondria, (iii) the mean pore dimension, or (iv) the number of pores per mitochondrion. The model was tested for transitions induced either by phenylarsine oxide (PhAsO) or by Ca2+ and Pi. Under both conditions, only a definite fraction of mitochondria was permeabilized, and the transition always followed a first-order reaction, indicating that mitochondria behaved as a homogeneous population. However, the equilibration of external solutes was rapid only in a fraction of permeabilized mitochondria and slow in the remaining fraction, due to restricted solute diffusion through narrower pores. With 0.2 mM PhAsO as the inducer, the fraction of permeabilized mitochondria was about 0.8. Sucrose diffusion was rapid only in 15% of this fraction (half-time less than 1 s) and restricted in the remaining 85% (half-time of about 60 s). Increasing PhAsO concentrations increased the number of pores per mitochondrion and the rate constant of the permeability transition, but not the mean pore diameter and the fraction of permeabilized mitochondria. With 0.28 microM free [Ca2+] and 1 mM Pi as the inducers, the fraction of permeabilized mitochondria was about 0.43. Sucrose diffusion was rapid in 60% of this fraction and restricted in the remaining 40% (half-time of about 9 s). The permeabilization process started above a threshold- free [Ca2+] of 0.15 microM, and the rate constant and the fraction of permeabilized mitochondria reached a maximum at about 0.2 microM, while both parameters were inhibited at higher free [Ca2+].