2,5-Bis(4-guanylphenyl)-1,3-oxazole, 2,5-bis(4-guanylphenyl)-1,3,4-oxadiazole and -1,3,4-thiadiazole, and 3,6-bis(4-guanylphenyl)pyridazine and several of their "cyclic guanyl" analogues have been synthesized. 2,5-Bis(4-guanylphenyl)-1,3-oxazole and -1,3,4-thiadiazole showed good activity, whithout acute toxicity, against Trypanosoma rhodesiense in mice, producing cures at a 3 mg/kg dosage level. This activity is comparable to stilbamidine, hydroxystilbamidine, and pentamidine in this test. In contrast, 2,5-bis(4-guanylphenyl)-1,3,4-oxadiazole shows a sharp reduction in activity in our test system. Generally, the cyclic guanyl analogues exhibit low orders of activity, and toxicity begins to appear at moderate dosage levels. All guanyl and cyclic guanyl compounds were synthesized from bisnitrile precursors by way of imidate ester hydrochlorides in a classical Pinner-type approach.