Pipecolic acid (PA, piperidine-2-carboxylic acid) is the major product of lysine metabolism in the mammalian brain (Giacobini et al., 1980). In this study we have characterized the binding of [3H]PA to P2 fraction membranes and its distribution in the mouse brain. The binding was found to be saturable (70 nM), temperature and Na+ and Cl- dependent. A high affinity binding site with an apparent KD of 33.2 nM and a Bmax of 0.2 pmol/mg protein was demonstrated. The regional distribution of [3H]PA specific binding in mouse brain showed the highest concentration in cerebral cortex, thalamus and olfactory bulb. Unlabeled PA (10(-3)-10(-11) M) displaced specific binding of [3H]PA in a concentration dependent manner. Out of several substances tested, only proline showed a similar pattern of displacement. Pre-incubation of the membrane preparation with GABA (10(-3)-10(-11) M) resulted in either an increase or decrease of [3H]PA binding depending on the concentrations of GABA and PA. These results suggest a modulatory action of GABA on PA binding sites. The postnatal development of [3H]PA specific binding was studied in the whole brain of the mouse. [3H]Pipecolic acid binding increased progressively (8-fold) from one day after birth to 16 days. Following this developmental peak, the binding decreased gradually to 30 days at which age, adult values were attained.