Styrylamidines are prepared by treating styrylsulfonylamidines with base. The styrylamidines are effective in the prevention of aggregation of blood platelets and as analgesics. Compounds of the invention are also useful as anticonvulsants, diuretics and antihypertensive agents. The styrylsulfonylamidines of the invention which serve as precursors to the styrylamidines also have analgesic properties. Illustrative of the styrylamidines of the present invention are 4-amino-N-(4-aminostyryl)benzamidine and N-(3,4-dichlorostyryl)acetamidine. An example of a styrylsulfonylamidine is N-(styrylsulfonyl)acetamidine.