Perfluoroisobutene (PFIB) is produced by the pyrolysis, and as a by-product during the manufacture, of polytetrafluoroethylene. When inhaled it produces a fulminating and sometimes fatal pulmonary oedema similar to that of phosgene after a latent period of 6-8 h. As part of a study to determine the retained dose and the factors that control the amount retained, this study has investigated the retention in rats of inhaled PFIB at concentrations of 10, 50 and 250 micrograms l-1 in a flow-through system combining head-only exposure and plethysmography. Uptake of PFIB was measured by gas chromatography during elevated and reduced inspired volume and respiratory rate induced by exposure to increased CO2 and injection of pentobarbitone, respectively. The percentage of PFIB retained in the upper airways and lungs was found to be 27.5, 28.1 and 23.7% of the amount inspired at the three concentrations tested. The rate of uptake (nmol min-1 kg-1) of PFIB was a power law of the amount inhaled, an n-fold increase in minute volume producing an nb-fold increase in uptake, where b varied between 0.4 and 0.85. Thus, doubling the inhaled dose produces a 1.3-1.8-fold increase in uptake with a corresponding decrease in percentage retained. The relative contribution of respiratory rate and tidal volume upon PFIB retention could not be defined.