AbstractThe correct structure of the reported 3‐aziridinyl‐6‐chloropyridazine was found to be 3‐[β‐(1‐aziridinyl)ethylamino]‐6‐chloropyridazine, which was confirmed by n. m. r. studies. The relative ease of nucleophilic displacement of a methylsulfonyl group versus a chloro group in equivalent positions on a pyridazine or a pyrimidine ring was compared. It was observed that the chloro rather than the methylsulfonyl group was preferentially replaced, as illustrated by substitution studies of 3‐chloro‐6‐methylsulfonylpyridazine and 4‐chloro‐