Fipronil is an insecticide/acaricide in the phenylpyrazole family. It acts by inhibiting the GABA complex, binding to the chloride channel and thereby blocking pre and post-synaptic transfer of chloride ions across the membrane. This results in uncontrolled activity of the central nervous system and death in insects and acarids.

*Absorption – After local application of the product in the cat, absorption of fipronil through the skin is negligible. *Distribution – After application, a concentration gradient of fipronil is set up on the fur of the animal extending from the point of application to the peripheral areas (lumbar zones, flanks,…). *Biotransformation – In ‘in vitro’ studies with subcellular liver fractions, the principal metabolite is the sulfone derivative of fipronil. However, this may be of limited relevance ‘in vivo’ as fipronil is poorly absorbed in the cat. *Elimination – The concentrations of fipronil on the hair decrease with time to attain a level of approximately 1 μg.g‑1 two months after treatment.