1. Uptakes of L-methionine and mannitol by rat jejunum in vitro were measured over test periods from 5 to 120 sec after 30 min pre-test periods in the presence or absence of Na.2. The initial stage in methionine uptake was dependent on the presence of Na(+) and to a lesser extent on the K(+) concentration. In contrast mannitol uptake was independent of Na and K.3. The initial stage in methionine uptake can be reactivated 30-60% within 5 sec by replacing an Na-deficient intestine into an Na-containing medium.4. Initial methionine uptake was greater with a normal intracellular and low medium Na concentration than with a high medium and low intracellular Na concentration. It is suggested that the intracellular Na concentration is a critical factor, more important than the Na gradient, in determining the rate of amino acid transfer across the luminal membrane.