The synthesis and anticonvulsant activity of a number of 2-aryl-5-guanidino-1,3,4 thiadiazoles are described. The unsubstituted guanidine 2a was found to possess potent anticonvulsant properties; considerable reduction or loss of activity however was observed with the majority of the substituted guanidines. Incorporation of the guanidine group into an imidazoline ring also resulted in a loss of activity. Secondary pharmacological evaluation confirmed the anticonvulsant properties of 2a but also revealed that the compound exhibited a considerable degree of sedative activity.