The effect of various dietary fats on the nucleoside triphosphatase (NTPase) activity and nuclear membrane lipid composition of rat liver during development was assessed. Rats fed a fat-free diet exhibited higher specific activity of NTPase at all ages, compared with control animals. In rats fed a sunflower oil diet, the specific activity of NTPase was also found to be highest at all ages than was observed in the control group. In contrast, animals fed the fish oil diet or peanut-rapeseed oil diet showed a decrease in NTPase activity in comparison with the control group. The specific activity of NTPase was correlated positively with dietary sigma PUFA n-6 (r = 0.03, p < 0.05) and negatively with the dietary sigma PUFA n-3 (r = -0.87; p < 0.05). The fatty acid composition of liver nuclear membranes of rats fed a fat-free diet revealed high levels of 16:1 n-9, 18:1 n-9, and 20:3 n-9 acids. A dramatic decrease in 18:2 n-6, 20:4 n-6, and 22:6 n-3 acids was observed. Animals fed a sunflower oil diet showed high levels of n-6 fatty acids, particularly 22:4 n-6 and 22:5 n-6, and low levels of monounsaturated fatty acids. However, when rats were fed a fish oil diet, the liver nuclear membranes were highly enriched in 20:5 n-3, and 22:6 n-3 acids, and there was a simultaneous decrease in arachidonic acid. From these observations it is concluded that dietary fats induce changes not only in the fatty acid composition of the nuclear membrane lipids but also in the specific activity of NTPase involved in nuclear function.