Δ2-Pyrazolines that do not have substituants in the 1- and 3-positions are not capable of isomerization to 3-substituted pyrazolines and can be converted to Δ1-pyrazolines by slow distillation in the presence of bases. This method, which is completely analogous to the synthesis of azo compounds from alkylhydrazones, gives good results when applied to 4-alkyl- and 5,5-dialkyl-Δ2-pyrazolines and makes it possible to obtain the corresponding Δ1-pyrazolines in yields of 40–70%. 4-Ethyl-, 5-methyl-5-ethyl-, and 5,5-diethyl-Δ1-pyrazolines are described for the first time.