A brief survey of the literature on gamma-glutamyl transpeptidase (gamma-GT) activity is included in this study. The levels of activity in the serum of normal Merino sheep (13,6-32,4 mI.U/ml) were ascertained as a preliminary to following the activity through the entire course of experimentally induced ovine lupinosis, a hepatotoxicosis caused by Phomopsis leptostromiformis (Kühn) Bubák. The response of the serum level of gamma-GT activity to the course of the disease was compared with that of glutamate oxaloacetic transaminase (GOT) and 2 liver function tests for the purpose of assessing its potential application in the study of this mycotoxicosis. Because the levels of activity of gamma-GT were more valuable for the early diagnosis of low grade acute intoxication and the detection of chronic liver involvement while those of GOT gave better information on the development of severe acute hepato-cellular damage, these 2 enzymes, considered together, were found to give the best information on the course of the toxicosis. The changes in gamma-GT activity during various stages of intoxication were also related to the histopathological lesions in the liver.