One hundred Osborne-Mendel rats were weaned at the age of 21 to 22 days, inoculated withStreptococcus mutansin the mouth, and fed a semisynthetic diet for the next 43 days. The control group received no treatment. The study groups received gel applications on their molars with placebo, chlorhexidine-fluoride (CXF), CXF plus 50ppm Sr, or CXF plus 250 ppm Sr daily for the first 21 days of the experiment. Although caries was significantly reduced by CXF and CXF plus 50 ppm Sr treatments, the Sr additive did not significantly improve the caries-preventive effect of CXF. The addition of 250 ppm Sr to the CXF gel seemed markedly to weaken the effect of CXF.