The triterpenoid azadirachtin was injected in adult house flies and the effect on the circadian rhythm of locomotor activity studied. In most cases (83%) period was increased (by 0.4 h, as a mean), whereas flies injected with the solvent of azadirachtin, ethanol, increased period in 22% only (by 0.3 h, as a mean). In 46% of the azadirachtin injected flies a splitting of the activity rhythm into two components with the same period length was induced, whereas only one of the ethanol controls (2%) showed splitting after injection. LiCl added to the drinking water changed the activity pattern from splitting to arrhythmicity in about 50% of the azadirachtin treated flies.