The effect of age on active transport of catecholamine precursors was studied in a healthy population of mature (12-mo.-old) and senescent (30-mo.-old) C57BL/6J male mice. No age differences were observed in uptake of L-(3H)-tyrosine IN VITRO by slices of hypothalamus, brain stem, and striatum at external concentrations equal to or greater than blood levels. However, age-related reductions were observed intermittently in the hypothalamus, brain stem, and striatum at very low concentrations of tyrosine (less than 10 pg/ml). No age differences were observed in incorporation of L-(3H)-tyrosine into hypothalamic proteins when corrected for intracellular uptake of L-(3H)-tyrosine. No age differences were observed in uptake of L-(3H)-DOPA. The possible contribution of age-related changes in extra-cellular space to decreased uptake was determined to be negligible by wash-out studies of tyrosine and by the uptake of inulin. A significant age-related increase of inulin uptake IN VITRO was observed under ionic conditions which minimize tissue swelling.