Between 1978 and 1981, 20 platypus (Ornithorhynchus anatinus) which had been held in exhibits for varying periods, were submitted for necropsy. The most common gross and histologic lesions were adrenal enlargement, pulmonary pathology consistent with shock or aspiration pneumonia, intestinal coccidiosis, the presence of trypanosomes, myocarditis and nephritis. Other conditions encountered included infestation with ticks (Ixodes ornithorhynchi), a mild infection of intestinal trematodes (Mehlisia ornithorhynchi), myocardial toxoplasmosis, and focal hepatic necrosis. Adrenal weights, both absolute and relative to body weight, were determined in 12 specimens, and used as parameters of each animal's response to the stress associated with captivity. The results showed that, in platypus held in captivity from about 1 day up to 6 mo, both parameters were higher than in animals which were examined within a few hr of capture. In view of the general lack of conclusive necropsy findings, it was considered that these results indicated that stress may have been a significant underlying factor in the death of these animals in captivity.