Our study aimed at investigating the impact of different levels (increased -10% and high -20%; w/w) and types (sunflower oil, rapeseed oil, cod liver oil, lard) of fat in the diet on the activity of the key enzyme in the corticosterone (Cs) biosynthesis pathway in adrenal cortex i.e. deoxycorticosterone 11β-hydroxylase (11β-OHase) (it was expressed as a percent of conversion of [¹⁴C]deoxycorticosterone into [¹⁴C]corticosterone during 2 h incubation) and on plasma corticosterone and leptin concentration (measured by RIA method) in rats. 102 male Wistar rats were fed their experimental diets for 3 or 6 weeks. All parameters were found to be significantly affected by dietary fat source. Rats fed the diet containing cod liver oil had the highest 11β-OHase activity, while those fed sunflower oil diet had the lowest. Sunflower and rapeseed oil diets brought about higher plasma leptin concentration than cod liver oil and lard diets. Plasma Cs concentration in rats fed cod liver oil diet was higher than in rats fed other fat diets. Additionally, diets containing 20% of fat caused higher enzyme activity than diets containing 10% of fat. There was no effect of fat level in diet on plasma Cs and leptin concentration. These effects were observed in rats fed for six weeks only for enzyme activity and in rats fed for three weeks for plasma Cs concentration. Moreover, 11β-OHase activity increased after 6 weeks of experiment, plasma Cs level - decreased after the same time.