5-(3-phenyl-3-oxo-propyl)-1H-tetrazole derivatives of formula (I) wherein each of R and R.sub.1, being the same or different, is hydrogen, halogen, hydroxy, trifluoromethyl, cyano, nitro, phenyl, benzyl, C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 alkoxy, C.sub.1 -C.sub.6 alkylthio, SOR.sub.4 or SO.sub.2 R.sub.4 wherein R.sub.4 is C.sub.1 -C.sub.6 alkyl, --N(R.sub.5 R.sub.6) in which each of R.sub.5 and R.sub.6 is, independently, hydrogen, C.sub.1 -C.sub.6 alkyl, formyl or C.sub.2 -C.sub.6 acyl; R.sub.2 is hydrogen, C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 alkoxy, benzyl, phenyl; R.sub.3 is hydrogen, hydroxy, C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 alkoxy, benzyl, phenyl or a group --N(R.sub.7 R.sub.8) in which each of R.sub.7 and R.sub.8 is, independently, hydrogen, C.sub.1 -C.sub.4 alkyl, benzyl, phenyl, or one of R.sub.7 and R.sub.8 is hydrogen and the other is COR.sub.9 wherein R.sub.9 is hydrogen, C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy, phenyl or a group --N(R.sub.10 R.sub.11) in which R.sub.10 and R.sub.11 are, each independently, hydrogen or C.sub.1 -C.sub.4 alkyl or, taken together, R.sub.2 and R.sub.3 form a carbocyclic C.sub.3 -C.sub.6 ring; and pharmaceutically acceptable salts thereof have kynurenine-3-hydroxylase enzyme inhibitory activity. ##STR1##