Rabbit muscle pyruvate kinase activity has been studied in the presence of L-phenylalanine and its analogs: L-phenylalanyl methyl ester (PheOMe), L-1-amino-2-phenylethyl phosphonic acid (PnPhe), L-alanine and L-1-aminoethyl phosphonic acid (PnAla). At appropriate pH and substrate concentrations all the analogs and Phe exhibited activatory and inhibitory effects at low (1--5 mM) and at high (above 5mM) ligand concentrations respectively. Activation of pyruvate kinase by Phe and PheOMe was observed at pH above 8.2 in the presence of 2.5 mM ADP and 0.5 mM phosphoenolpyruvate (P-pyruvate), while PnPhe activation was also observed at pH 7.5. The activatory effect followed the order: PnPhe much greater than Phe greater than PheOMe. All the effectors showed a mixed type of inhibition or activation with P-pyruvate as a variable substrate and a non-competitive inhibition or activation with ADP as a variable substrate.