The effect of exercise and/or high fat diet on carnitine status and carnitine palmitoyltransferase I(CPT I) level were investigated in Weanling Sprague Dawley rats. The rats were fed an AIN 76 diet or a modified high fat AIN diet, supplemented with 35% corn oil, for 31 days. During the 31 day period half of the animals in each dietary group were exercised on a treadmill for 90 minutes per day. Carnitine concentrations were determined in plasma and liver and CPT I mRNA levels were measured by Northern blot analysis with CPT I cDNA probe in livers of rats. Exercise rats gained less weight than non exercised rats during the study for high fat diet group. Exercise rats had a higher plasma acid soluble acylcarnitine and acid insoluble acylcarnitine concnetrations than non exercised rats for normal diet group. Exercise or high fat diet increased liver carnitine concentration, but a mixed effect was not shown. In exercised rats, CPT I mRNA levels increased significantly relative to those of nonexercised rats. CPT I mRNA levels also increased when compared high fat fed rats with those of normal diet fed rats. These data suggest that there is a correlation between carnitine concen trations and CPT I mRNA levels and that CPT I can be regulated at the transcriptional level by exercise and/or high fat diet.