The tendo Achillis of man and its homologue from a fairly wide range of exotic animals, most of which had died in captivity from natural causes, have been examined for stainable lipid. Adiposity of the tendon was seen principally in man. Chondroid plaques, containing an abundance of intracellular triglycerides, are a normal feature in birds. Similar thickenings in the few reptiles examined were rich in phospholipids. Extracellular (perifibrous, interstitial) deposits of lipid with a high content of esterified cholesterol were found in more than 90% of human specimens, in 52% of other mammals and in 62.5% of birds included in the survey. In general, lipid insudation of tendons increased with age and was less intense in animals than man. Aetiological factors and the relationship of extracellular lipid deposits in tendons to those in the arterial wall and dura mater are considered.