The mechanisms involved in the inhibitory effects of antilipolytic agents on rat liver peroxisomal fatty acid oxidative activity have been explored. Treatment of fasting rats with antilipolytic drugs (either 3,5-dimethylpyrazole (12 mg/kg body weight) or Acipimox (25 mg/kg body weight] resulted in a decrease in free fatty acid and glucose plasma levels within 5-10 and in a significant increase in the plasma glucagon to insulin ratio within 15. Changes in the fatty acid oxidative activity appeared with a 2.5-3 h delay and were then very rapid (a 30-40% decrease in the activity occurred in additional 2 h). Many peroxisomal enzyme activities (including non-beta-oxidative activities such as uricase and D-amino acid oxidase) exhibited similar changes with the same delay. Simultaneously with the enzyme changes, at the electron microscope level many autophagic vacuoles were detected in the liver cells, often containing peroxisomal structures. Glutamine, an inhibitor of proteolysis in vivo, prevented the decrease in enzyme activities. It was concluded that the decrease in peroxisomal enzyme activities may be the consequence of enhanced peroxisome degradation due to the stimulation of autophagic processes in liver cells.