Diclazuril, 2,6-dichloro-α- (4-chlorophenyl)-4- (4,5-dihydro-3,5-dioxo-1,2,4-triazin-2(3H)yl) benzeneacetonirile, is an anticoccidial drug, marketed as Vecoxan® for oral use in lambs. Its mechanism of action blocks the excretion of oocysts interrupting the life cycle of the parasite. Because of alleged species specific differences, the aim of the present study is to evaluate the pharmacokinetic of diclazuril in ruminant and pre-ruminant lambs. The drug was orally administrated to ruminant (n=10) and pre-ruminant (n=10) lambs with a dose of 5 mg/kg. Blood samples were collected from 0 to 120 hours after drug administration. Plasma extracts were analyzed using a new HPLC method. The linear concentration range for diclazuril analysis was 0.1-5 μg/ml (r2=0.999). The plasma concentrations versus time curves were fitted with a noncompartmental method. Significant differences between the groups were found in the concentration versus time curves and also in the pharmacokinetic parameters. This appeared to be related to age differences of the lambs. In conclusion, these findings could be useful to determine a clinical dose of diclazuril in this animal species.