A novel structural class of highly potent and selective 5-HT3 receptor antagonists is described. The compounds in this new series contain a thiazole moiety linking an aromatic group and a nitrogen-containing basic region; the thiazole group appears to be acting as a carbonyl bioisostere in this system. An optimized member of this series, 4-(2- methoxyphenyl)-2-[[4 (5)-methyl-5 (4)-imidazolyl] methyl] thiazole (5), exhibits oral activity ...