Three groups of adult rats were fed black currant or olive oils or a 1:1 mixture of the two for three months. Feeding black currant oil diets, high in 18:2 (n-6), 18:3 (n-6), 18:3 (n-3), increased the heart and liver plasma membrane content of linoleic and arachidonic acid with a concomitant decrease of oleic acid. PUFA, n-3 and n-6 content and the bilayer lipid fluidity as examined by measuring the fluorescence anisotropy of diphenylhexatriene were not significantly affected. On the contrary, the 5'-nucleotidase of liver membrane of rats fed black currant diets was lower than that observed in membranes of liver from olive oil fed rats. Therefore it is concluded that PUFA and n-3/n-6 ratio as well as membrane fluidity do not influence the 5'-nucleotidase activity. It is suggested that the activity is sensitive to the amount of a specific fatty acid of the membrane (i.e., oleic or arachidonic acid) and/or to lipid supplementation which can influence the eicosanoids metabolism.