Abstract: By the use of ANS(1-anilinonaphthalene- 8-sulfonate) and DPH( 1,6- diphenyl-1, 3, 5-hexatriene)as fluorescent probes, correlation between liver microsomal membrane fluidity and drug metabolizing enzyme activity has been studied in rats. phenobarbital(PB) ip treatment caused an increase in P-450 content, cytochrome C reductase.amiropyrine N-demethylase(AM D)and glutathione S-transferase(GST)activities by 78, 66,270 and 52%, respectively. However, there was a simultaneous decrease in microsomal membrane fluorescent intensity and microviscosity, i. e. an increase in membrane fluidity. There is a positive linear correlation between microsomal membrane fluidity and cytochrome C reductase and AMD activities (r= 0. 798, r= 0. 781, respectively, P<0.05). This result suggests that there may be some relationship between microsomal membrane fluidity and drug- metabolizing enzymatic activities in PB-treated rats.