1. pharmacological ranitidine can competitively block the binding between histamine and H2 receptor. The inhibitory effect on gastric acid is 5 times to 12 times that of cimetidine. Therefore, it is an effective H2 receptor blocker. 2. LD 501440 ~1750 mg/kg was taken orally by toxic mice. The maximum daily non-toxic dose for rats (males) is 500mg/Kg, for rats (females) is 250mg/Kg, and for dogs is 40mg/Kg. The maximum daily non-toxic dose for rats at 26 weeks was 100mg/Kg, and that for dogs was 40mg/Kg. Mice 100 ~ 200mg/kg orally 1 14 weeks, rats 100 ~ 200mg/kg orally 129 weeks. No carcinogenic effect was found.