Serial blood samples were collected over a 3·5–8 h period from ten adult male macropodid marsupials belonging to five different species to study the pattern of testosterone secretion. The concentration of testosterone in the plasma ranged from 0·5 to 9·5 ng/ml, and in each animal the levels declined progressively during the study; this effect was attributed to the stress effects of handling. Injection of 5 μg synthetic luteinizing hormone releasing hormone induced a rapid and substantial increase in the level of testosterone.