In order to study the effects of arsenic on fluorosis and interaction between arsenic and fluoride, nuclear proton microprobe technique and quantitative histological method were used to observe the distribution of arsenic and fluoride in teeth and their effects in rats fed with water containing arsenic (30 mg/L) and fluoride (100 mg/L). Results revealed (1) fluoride could cause dental fluorosis in rats, similar to that seen in human beings, and characteristics of fluorosis was evaluated by a grading system; (2) no detectable amount of arsenic was deposited in dentine, and no damage to teeth could be seen; (3) no obviously combined effects of arsenic and fluoride on dental tissues were seen, and the toxic effect of fluoride was not influenced by arsenic; and (4) a synergistically inhibitory effect of both arsenic and fluoride on body weight was found.