A naphthyloxyacetic acid derivative of the formula (I) ##STR1## wherein R.sup.1 is H, alkyl, alkylene-(--COOR.sup.10, --OH, --CONR.sup.4 R.sup.5, --CONR.sup.6 -alkylene-OH, --NR.sup.4 R.sup.5, -cyano or -tetrazolyl); A is single bond, alkylene, alkenylene, --S-alkylene, --O-alkylene; B is NR.sup.3 CO, CONR.sup.3 ; R.sup.2 is (1) alkyl (2) alkenyl, (3) alkyl or alkenyl substituted by 1-3 of phenyl, cycloalkyl, naphthyl and heterocyclic ring containing nitrogen atom (the said ring may be substituted by 1-3 of alkyl, alkoxy and halogen etc.), (4) NR.sup.7 R.sup.8 or (5) alkylene-NR.sup.7 R.sup.8 ; non-toxic salt thereof, non-toxic acid addition salt thereof and hydrate thereof can bind the PGE.sub.2 receptor and exhibits the activity to antagonize or agonize for PGE.sub.2., therefore, they are useful as PGE.sub.2 antagonist or PGE.sub.2 agonist.