The effectiveness of three techniques to deliver a diazo dye suspension into the lungs of rats was compared. The intratracheal nebulization (ITN) technique delivered 10 microl of the suspension per 5-ml puff of air in 10 puffs as an aerosol. The intratracheal fast instillation (ITFI) technique delivered 100 microl of the suspension in a single 2-ml puff of air as droplets. The nose-only inhalation (NI) technique aerosolized the suspension at an analytical concentration that provided a calculated dose equivalent to 100 microl of the suspension in a 2-h inhalation period. Immediately after dosing, all the rats were killed by exsanguination. The trachea was tied and the lung was inflated in situ with air. After fixation, 5-microm thick slices were prepared from each lobe of the lung at a plane perpendicular to the axis of the lobar bronchus at levels proximal, medial and distal to the hilus. The numbers of bronchi, bronchioli and alveolar ducts within four ranges of diameters and the proportion of each selected area of lung tissue with and without dye particles were quantified using electronic imaging analyzers. The results indicated that ITN and ITFI dispersed the particles evenly throughout most of the airways and in patches in the alveoli. The NI technique dispersed the particles homogeneously throughout the airways and the alveoli in the lungs. The mean number-percentage and the mean area-percentage data revealed that the doses delivered by ITN and NI were approximately 60% and 10%, respectively, of the ITFI dose. Thus, the ITFI technique appeared to be most suitable for pulmonary absorption and disposition studies where dosage precision is of primary concern. The ITN technique would need further improvement to meet the requirements for dose precision and particle distribution. For both ITFI and ITN, particle size was apparently not a critical determinant for deposition. The NI technique is suitable for inhalation toxicity studies where the pattern and uniformity of particle deposition is the primary concern.