Dose (mg/kg b. w.) Total number of animals Number of dead animals z d ∑(z.d) 200 8 0 - - - 400 8 1 0.5 200 100 600 8 2 1.5 200 300 800 8 4 3 200 600 1000 8 6 5 200 1000 1200 8 8 7 200 1400

z: mean number of dead animals in two successive doses d: constant factor between two successive doses LD50 = the biggest dose which kill all animals - Σ(z.d)/n = 1200-3400/8 = 775 mg /kg b. w. By using the conversion table of Paget and Barnes (1964), LD50 for rats was calculated from that of mice and was found to be 542.5 mg /kg b. w. 1/50 of LD50 is about 10 mg /kg b. w. which was considered as sublethal dose that used as therapeutic dose in the subsequent work

Table 3: Determination of LD50 of pyrimido[1,5-a]quinazoline derivative (4b) in albino mice.