The geometrically isomeric cis- and trans-5-3-methyl-4-ethyl-3-carbomethoxy-Δ1-pyrazolines were prepared and their pyrolysis and photolysis studied. The isomeric pyrazolines gave cyclopropane products of mixed stereochemistry, but predominantly with retention of the stereochemistry present in the starting pyrazoline. Each pyrazoline gave stereospecifically a different α,β-unsaturated ester product. This stereospecificity can be related to the structure of the pyrazoline through the requirement that the loss of nitrogen must be from the side of the pyrazoline that is trans to the hydrogen at C-4.