The pulmonary toxicity of particles is often studied using a single intratracheal instillation of the material. It was hypothesized that smaller multiple intratracheal administrations of silica would result in differences in pulmonary responses as compared to a single large intratracheal administration. In the first of a series of experiments, the pulmonary responses in male F344 rats to a single intratracheal instillation of crystalline silica (5 mg/100 g body weight) given on d 0 were compared with those resulting from 5 consecutive daily intratracheal administrations of the dust (1 mg/100 g body weight/d) with the initial dose given on d 0. Controls received saline intratracheally. In the second experiment, the dose was reduced to 1 mg/100 g body weight for the single-dose protocol and 0.2 mg/100 g body weight/d for 5 consecutive days for the multiple-dose protocol. In both experiments, responses were assessed on d 14. In the third experiment, the doses were the same as the first experiment, but the responses were assessed on d 28. The indices of toxicity were cellular differentials recovered by bronchoalveolar lavage, which is an index of inflammation, and the level of albumin in the bronchoalveolar lavage fluid, a measure of damage to the capillary-epithelial barrier. At the higher dose of silica, similar levels of inflammation and lung damage were evident in both dosing protocols. Less severe responses occurred at the lower dose. The comparative pattern between the single and multiple dosing protocols was similar in all three experiments. Since only minor differences were noted in the pulmonary responses when the responses to the single- and multiple-dose protocols were compared, data indicate that the multiple-dose protocol does not offer any advantages over the single-dose protocol.