Caries scores were examined in rats being fed a high sucrose diet and inoculated with Streptococcus mutans (strain No. 6715) in order to determine if addition of NH4F to the drinking water would afford the same caries protection as addition of NaF. Sixty rats were divided into six groups. Group I received distilled water, II NaF, and III NH4F. Groups IV and V received NaF or NH4F with Na- or NH4-phosphate, respectively, while group VI received NH4-phosphate alone. The F concentration was 30 ppm. The PO4 concentration was equimolar with that of F (1.6 mM). All solutions were prepared at pH 7. Another 60 rats were similarly treated except that all solutions contained 0.2% sucrose and 16 mM acetic acid adjusted to pH 5 with NaOH. The mean caries scores ( ± SD) in the pH 7 series were for group I 55 ± 23, II 17 ± 24, III 5 ± 6, IV 7 ± 9, V 9 ± 22 and VI 43 ± 19. The corresponding caries scores in the pH 5 series were for group I 37 ± 24, II 15 ± 17, III 6 ± 6, IV 6 ± 5, V 1 ± 1 and VI 59 ± 27, respectively. NaF and NH4F were equally caries-protective at pH 7 irrespective of the presence of PO4. At pH 5, NH4F was more protective than NaF, with or without PO4. Addition of PO4 to either NH4F or NaF resulted in increased caries protection. The best protection was afforded by NH4F with or