In a recent publication we reported potent antitrypanosomal activity for a series of substituted 2,5-bis(4-guanylphenyl)furans and related "cyclic amidines". These furan compounds were envisioned as potential antiprotozoan agents based upon structural analogy with biologically active aryldiamidines and upon their potential to interact with DNA as their bioreceptor. In addition to expected antitrypanosomal activity, it was hoped that these compounds might also exhibit antimalarial activity in view of the reported activity of diminazene and pentamidine against Plasmodium vinckei. In view of the potent activity observed for the 2,5-bis(4-guanylphenyl)furans, we have prepared and evaluated the analogous thiophenes and N-methylpyrroles and the results of these efforts constitute this report. 2,5-Bis(4-guanylphenyl)thiophene and 2,5-bis(4-guanylphenyl)-N-methylpyrroles and several of their "cyclic amidine" analogues have been synthesized. The thiophenes and pyrroles were synthesized by treatment of 1,4-bis(p-bromophenyl)-1,4-butanedione with H₂S-HCl or CH₃NH₂-HOAc, respectively, to produce the corresponding 2,5-bis(4-bromophenyl)thiophene and 4-methylpyrrole. The dibromophenyl compounds were converted into the corresponding bis-nitriles by reaction with Cu₂(CN)₂. The latter compounds were converted by way of imidate esters to the guanyl and "cyclic guanyl" targets. Their antimalarial and antitrypanosomal activity has been assessed. None of these compounds showed significant antimalarial activity; however, all displayed good levels of activity against Trypanosoma rhodesiense in mice. 2,5-Bis(4-guanylphenyl)thiophene and 2,5-bis(4-guanylphenyl)-N-methylpyrrole produced cures in mice at the ~1 mg/kg dosage level. These two compounds are of comparable activity to stilbamidine, hydroxystilbamidine, and pentamidine in this test. The "cyclic amidines" generally exhibited lower antitrypanosomal activity than their guanyl counterparts. In light of the results reported here and our previous results, we conclude that "cyclic amidines" exhibit lower orders of antitrypanosomal activity than their true guanyl analogues and that acute toxicity is generally encountered with the cyclic guanyl compounds, particularly at higher dosage levels. The two most active compounds, 6 and 10, exhibit comparable levels of activity to the standard diamidines, 14-16, and they are approximately as effective trypanocides as their furan counterparts. The fact that the thiophene, N-methylpyrrole, and furan systems show comparable activity suggests that the role of the five-numbered ring heterocycle in these compounds could be nothing more than a relatively inert spacer for the guanylphenyl functions. However, our earlier work on the furan system shows that enhanced activity is found by substitution on the furan ring. These observations were attributed to distribution differences and/or differences in binding to the bioreceptor. A study to determine if binding to DNA (the possible bioreceptor) can be related to the structural variations in these substituted furan, thiophene, and N-methylpyrrole systems is underway. Because the activity of the thiophene and pyrrole derivatives reported here did not surpass that of their furan analogues, a study of the effect of substitution on these heterocyclic ring systems was not undertaken.