The lung retention of inhaled 57Co labelled Co3O4 particles was measured in rats whose ages at exposure ranged from 3 to 46 weeks. Significantly lower lung retention was found in the younger animals and this could be attributed to a faster rate of translocation of 5Co from the lung to blood. Mechanical clearance rates were similar for all age groups. Comparison of these results with those of other studies suggest that mechanical clearance rates are independent of the chemical form of the material inhaled. The results of this study suggest that in the extrapolation of the results of animal experiments to humans, due consideration should be given to age related changes.