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The preparation of a series of thirty -five pyrazolines and two anomalous compounds (the latter obtained from cinnamylideneacetophenone and cinnamylidenepinacoline, respectively), twenty -five of which are believed to be new, is described. Most of these pyrazolines fluoresce in the solid state on exposure to ultraviolet rays. Many fluoresce in daylight when dissolved in organic solvents. Of those which exhibit no visible fluorescence in daylight, many fluoresce in ultraviolet rays; those which fluoresce in daylight do so with increased brilliance on ultraviolet irradiation. The behaviour of the members of the series in alcoholic, chloroform and benzene solution, on exposure to ultraviolet rays, has been investigated.