The effect of fasting on Δ5 desaturation activity in Minimal Deviation Hepatoma 7288c (HTC) cells and in rat liver microsomes was studied. When HTC cells were incubated in the absence of carbon sources in the medium (fasted cells) the conversion of [1-14C] eicosa-8,11,14-trienoic acid to arachidonic acid increased significantly compared to those cells incubated in complete medium (fed cells). This effect was also shown in rat liver microsomes of rats fasted for 12, 24 and 48 hr. Fasting produced a decrease of the amount of ethanolamine fraction in HTC cells together with a significant increase in the specific activity of the same fraction. Fasting also modified the fatty acid composition of HTC cells and rat liver microsomes. There was observed a relative increase of 18:2ω6 acid. The importance of the difference in fatty acid and lipid composition among fasted and fed HTC cells and rat liver microsomes on Δ5 desaturation activity was discussed.Facultad de Ciencias MédicasConsejo Nacional de Investigaciones Científicas y Técnicas