The thermal isomerisation of the pyrethrins to isopyrethrins has been studied, the progress of the reaction being followed by the rise in optical density at 2700 A and the fall at 2300 A. The isomerisation is a first-order reaction with Io6k equal to 4.58 and 481 at 125' and 195' respectively ; these values give E* = 24,610 cal. mol.-l ; log A = 8.17 ; AS+ = -21.8 cal. deg.-l.In normal (unheated) pyrethrum extract the ratio of the optical densities a t 2700 A and 2300 A is 0.08 ; in pyrethrum extract which has been fully isomerised by heat this ratio is 0.62.The biological activities of normal and of partially and completely isomerised pyrethrum extract have been examined by three methods (two on flies and one on grain weevils). Extract in which the pyrethrins have been completely isomerised to isopyrethrins has approximately one-half of the lethal, and one-quarter of the knockdown, activity of normal extract on houseflies.Pyrethrum extract which has been submitted to heat treatment shows no loss in biological activity if the ratio of the optical density a t 2700 A to that at 2300 A has not risen above 0.095. IntroductionThe structural alteration of the pyrethrins effected by heat was first observed by Staudinger 8r Ruczicka,l and confirmed by Haller & LaForge2 who reported that the biological activity of pyrethrum extract decreased when the material was subjected to prolonged heating.

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