Cycloaddition reactions of diazomethane with esters of 2 .. propenoic acids, having an aromatic or heteroaromatic substituent at position 3, have been described. These reactions give rise to esters of 4-aryl-2-pyrazoline-3-carboxylic acids 1- 3 or 4-hetero- aryl-2-pyrazoline-3-carboxylic acids4 ,s. We thought it worth while to ~tudy the reactions of diazomethane with methyl esters of trans-2-pyridine-, 3-pyridine- and 4-pyridine-2-propenoic acids (I a, I c, I d), with ethyl 2- ...