Abstract: The title compound was synthesised by the reduction of L-phenylalanine with lithium in liquid ammonia. Consistent with the Birch rule the main product was 2,5-dihydro-L-phenylalanine. 1,4-dihydro-L-phenylalanine was isolated by HPLC on a reversed phase column and characterised as a minor product of the Birch reduction. Both dihydro-L-phenylalanines were assayed in the phenylalanine ammonia-lyase (PAL) reaction. 2,5-Dihydro-L-phenylalanine was a substrate for PAL with Km about one third and Vmax 30 times less than that of the values of the natural substrate L-phenylalanine and converted into the corresponding 2,5-dihydrocinnamic acid. On the other hand 1,4-dihydro-L-phenylalanine was not converted by PAL. Both isomers were, however, competitive inhibitors of the enzymatic reaction. The Ki value of 1,4-dihydro-L-phenylalanine was 5.1 times higher as with the 2,5-dihydro isomer. These results support the proposed mechanism of the PAL reaction starting with an electrophilic attack of the 3,5-dihydro-5-methylidene-4H-imidazol-4-one (MIO) prosthetic group at the ortho position of the phenyl ring of the substrate.