Lung deposition of fibrous aerosol was studied by using a model of the third lung bifurcation with dimensions based on the symmetric model A of Weibel. A newly developed breathing simulator was attached to the bifurcating tubes to yield cyclic flow. The fractions of fibers that deposited in cyclic flow conditions (respiration rate : 15 and 30 breaths/min) were compared with those in steady flow conditions. Glass fibers that deposited in the daughter tubes and those which escaped from the tubes were observed under a scanning electron microscope (SEM) and they were divided into three classes according to their length, i. e., 10-20 μm, 20-40 μm and 40-80 μm. The deposited fractions were calculated for each class. For some of fibers depositing in the tube and those escaping from the tube, both length and diameter of fibers were measured and the joint length-diameter distribution of fibers was expressed by a bivariate lognormal distribution. The diameter distributions for each range of fiber length were used to determine the fractions of deposited fibers ranging 1 to 2 μm in diameter. The results indicated that the deposited fractions increased as fiber diameter and length increased as well as respiration rate. In particular, deposited fractions of small diameter fibers were strongly influenced by the length. The values of deposited fractions at 30 breaths/min were 1.4 to 1.7 times those at steady flow with the same time average flow rate, and the values at 30 breaths/min were much smaller than those calculated by Harris's theoretical equation.