The determination of chlorhexidine in pharmaceutical formulations is carried out using flow injection analysis (FIA) with measurement by atomic absorption spectrometry (AAS). The method is based on the formation of a copper-biguanide complex precipitate when the sample is injected into an ammoniacal copper solution. The precipitate is retained on a plastic or paper filtering device. A nitric acid stream dissolves the precipitate and carried the Cu(II) to the AAS detector. The chlorhexidine is determined over the range 5-20 ppm. The influence of interfering substances is investigated.