New heterocyclic derivatives of GABA, with a GABA moiety on the 3 position of a 2-imino-1,3,4-thiadiazole ring, have been readily prepared by alkylation of 2-amino-5-aryl-1,3,4-thiadiazoles in dimethylformamide. They are active as GABAA antagonists, inhibiting the action of GABA on the guinea-pig ileum about 5 times less potently than bicuculline.