Immediately after a 20 min nose only exposure to 51Cr labeled polystyrene latex microspheres (1.4 MMAD, geometric standard deviation = 1.3, 2 micrograms m-3), 23% of the measured radioactivity was within the trachea-lungs of the exposed rats, 37% was within the gastrointestinal tract, 10% was within the defurred head, and 30% was on the fur. One hour after deposition these percentages had changed to 20, 64, 5 and 11%, respectively. As a result of this non-pulmonary particle load transiting the esophagus, external thoracic radioactivity measurements did not accurately reflect excised lung measurements until 30 h post-deposition. Consequently, it was necessary to combine excised lung ratioactivity data (0-30 h post deposition) with serial external thoracic measurements (30-934 h post-deposition) to measure the clearance of these microspheres. Use of an annulus shaped detector sufficiently increased sensitivity to allow measurements to be made through at least 934 h post-deposition. The lung radioactivity retention curve was biexponential with half times of 34.3 +/- 2.3 and 963 +/- 107 h. Simultaneous exposures to 5% CO2 increased deposition by 40% but failed to reduce deposition inhomogeneity and test variability.