Both mono-and dicotyledonous species catabolize putrescine to -r-aminobutyric acid (GABA), but by two different pathways. GABA is the major labeled product in pea shoots and oat leaves fed with a 2-4h pulse of [1,4-14C]-putrescine (Put) or [1,4tetramethylene-~*C]-spermidine (Spd), respectively. In the presence of 1-10 ~M gabaculine, a specific inhibitor of GABA:pyruvate-transaminase, the label appearing in GABA increases 2 to 7-fold, which indicates that the transamination reaction is a major fate of GABA formed from Put or Spd in vivo. The conversions to GABA were demonstrated in vitro in coupled assays involving diamine oxidase from pea or polyamine oxidase from oat, and pyrroline dehydrogenase (PYRR-DH). The latter enzyme from either pea or oat is strictly NAD-dependent and is specific for pyrroline. The optimal temperature (40-45 Β°C) and pH (7.5-8.0) are similar to those of bacterial PYRR-DH. In all cases the enzyme was inhibited by the NAD analogs thionicotinamide and aminopyridine dinucleotide (0.1-1.0 mM). In addition to pea and oat, PYRR-DH was also detected in corn, barley, soybean and broadbean. Di-and polyamine oxidase are released by enzymes which degrade the cell wall, while PYRR-DH remains associated with the protoplast.