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Manganese (Mn) and cadmium (Cd) profiles in olfactory bulbs of California ground squirrels (Spermophilus beecheyi) trapped at Lawrence Livermore National Laboratory's Site 300 facility in California were determined with proton induced X-ray emission (PIXE). As a reference, Mn profiles in olfactory bulbs from laboratory rats exposed via nose-only inhalation to 0.53 mg/m3 Mn in the form of MnCl2 were also determined with PIXE. Atomic absorption spectrophotometry was used to measure soil Mn and Cd contents from the trapping sites and Mn and Cd contents in ground squirrel liver and leg muscle tissues. The data from laboratory rats revealed that Mn uptake into the olfactory bulb occurs via inhalation exposure. Data from ground squirrels and knowledge of the collection sites indicate that although several routes of exposure may occur, fossorial rodent olfactory uptake affords a significant exposure route to Mn and Cd in soils. Measured biotransfer factors (ratio of leg muscle tissue metal content to soil metal content) for Cd in ground squirrels were 10(3)-fold greater than exposure modeling estimates based on oral Cd uptake data from livestock. The measurements for ground squirrel tissues show that when conducting ecological risk assessments for natural habitats considerable care should be taken in selecting transfer factors. Specifically, transfer factors derived from data pertaining to comparable exposure pathways and ecological setting should be used wherever possible.