Abstract: A physiologically acceptable, radiolabelled compound of general formula (I) consisting of a compound of the ##STR1## where k is an integer from 2 to 5l is an integer from 1 to 5X and Y are independently selected from phenyl, naphthyl, pyridine or quinoline radical, each having at position 2 one of --OH, --SH, --NH.sub.2 or --COOH and each of X and Y can be optionally substituted by one or more amino, halogen, hydroxy, mercapto, nitro, cyano, thiocyano, alkyl, alkoxy, halogenoalkyl, acyl, acylamino, acyloxy, carboxyl, alkoxycarbonyl, carbamoyl, pyridoylamino, N-carboxyalkyl-carbamoyl, sulpho, sulphamoyl, mono- or dialkylated or phenylated sulphamoyl which can also carry one or more substituents R', alkylsulphonyl, alkoxysulphonyl, or by an optionally hydroxy-containing phenylsulphonyl or phenoxysulphonyl; where R' is as defined for X and Y; or pharmaceutically acceptable salts thereof.