The penetration of fluoride and chlorhexidine ions in dental plaque was investigated in vitro, employing small cylinders of plaque collected in vivo. It was found that 50–60 nmol of fluoride from a 2% neutral solution of NaF penetrated 2 mm of plaque in 3 min but that further penetration occurred after 9 min. The experiment may indicate that the traditional removal of plaque before application of 2% NaF is not necessary. Of 0.2% chlorhexidine 0.40–0.50 nmol was found up to 2 mm inside the plaque. This concentration did not increase significantly over a period of 3 min.