The thermal conversion of 4-isoxazolines to 4-oxazolines involves the transposition of two ring members. The ring-contraction and ring-expansion sequence in the reaction 25 has been previously clarified. The low N−N bond energy should favor an analogous conversion of 3-pyrazolines 6 to 4-imidazolines 7; the first example of such a transformation is reported here. In the yellow 16, the 3-pyrazoline