arylpiperazinylalkylpyridazinones structurally related to the previously described lead A (5-[4-(3-chlorophenyl)piperazin-1-yl]-propyl}-3-methyl-7-phenylisossazolo[4,5-d] pyridazin-4-(5H)-one) were synthesized and tested for their analgesic activity. Many of the tested molecules, at the dose of 20 mg kg-1 p.o., showed high antinociceptive activity, in particular, compounds 5a, 11c, 15a, 21 and 22, which were