Glutamine metabolism in the liver is essential for gluconeogenesis and ureagenesis. During the suckling period there is high hepatic protein accretion and the portal vein glutamine concentration is twice that in the adult, whereas hepatic vein glutamine concentration is similar between adult and suckling rats. Therefore, we hypothesized that glutamine uptake by the liver could be greater in the suckling period compared to the adult period. The present studies were, therefore, designed to investigate the transport of glutamine by plasma membranes of rat liver during maturation (suckling--2-week old, weanling--3-week old and adult--12-week old). Glutamine uptake by the plasma membranes of the liver represented transport into an osmotically sensitive space in all age groups. Inwardly directed Na+ gradient resulted in an "overshoot" phenomenon compared to K+ gradient. The magnitude of the overshoot was greater in suckling rats plasma membranes compared to adult membranes. Glutamine uptake under Na+ gradient was electrogenic and maximal at pH 7.5, whereas uptake under K+ gradient was electroneutral. Glutamine uptake with various concentrations of glutamine under Na+ gradient was saturable in all age groups with a Vmax of 1.5 +/- 0.1, 0.7 +/- 0.1 and 0.5 +/- 0.06 nmoles/mg protein/10 seconds in suckling, weanling and adult rats, respectively (P < 0.01). Km values were 0.6 +/- 0.1, 0.5 +/- 0.1 and 0.5 +/- 0.1 mM respectively. Vmax for Na(+)-independent glutamine uptake were 0.6 +/- 0.1, 0.55 +/- 0.07 and 0.54 +/- 0.06 nmoles/mg protein with Km values of 0.54 +/- 0.2, 0. +/- 0.1 and 0.5 +/- 0.2 mM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)