We report nineteen 4‐aryl‐ and 4‐arylalkyl‐1‐phthalazinamines (5—8) which we prepared and tested for antithrombotic properties. All compounds were assayed for their antiplatelet activity in the “Born test” with collagen as inducer of the aggregation. N‐[4‐(1H‐1, 2, 4‐triazol‐1‐yl)butyl]‐4‐phenyl‐1‐phthalazin‐amine (7 c) was the most potent compound, having an IC50 of 8 μM. When 5‐HT (Serotonin) was used to start aggregation the N‐(furan‐2‐yl‐methyl)‐4‐phenyl‐1‐pthtalazinamine (8 a) had an IC50 of 2 μM. In vivo potencies were highly significant. N‐[5‐(1H‐1, 2, 4‐triazol‐1‐yl)pentyl]‐4‐phenyl‐1‐phthalazinamine (7 d) inhibited thrombus formation by 12% (P < 0.002) in arterioles and 7% (P < 0.01) in venoles as tested with our laser thrombosis model. For compound 8 a we surprisingly found an antagonism to the 5HT2A receptor, which is most likely the mechanism of the inhibition of aggregation by this compound.