ABSTRACT Ranitidine is a histamine H 2 ‐receptor antagonist commonly used to treat gastric ulceration in horses. The author's laboratory conducted a study some years ago in the early 2000s on its metabolism as well as its urinary elimination profile in two geldings. With the technology advancement as well as popularity of blood for doping control testing, the laboratory has recently conducted another administration trials of the substance in six horses to study the in vivo metabolism of ranitidine, aiming to identify and reinvestigate the appropriate target(s) for controlling misuse of ranitidine in horses as well as to study its elimination in blood. To study the elimination and biotransformation of ranitidine, administration experiments were performed by giving six castrated horses (geldings) each 25 mL of Ulcerguard oral paste (equivalent to 9.8 g of ranitidine) in the morning and 20 mL of oral paste (equivalent to 7.9 g of ranitidine) in the afternoon daily for eight consecutive days. The postulated in vivo metabolites included ranitidine‐S‐oxide (M1), ranitidine‐N‐oxide (M2), desmethylranitidine (M3a/b) and furoic acid analogue of ranitidine (M4), resulting from oxidation, demethylation and oxidative deamination of ranitidine. To control the misuse of ranitidine in horses, elimination profiles of urinary and plasma ranitidine were established. Free ranitidine was detectable for at most 8 days and 72 h in urine and plasma, respectively. Both metabolites ranitidine‐S‐oxide and ranitidine‐N‐oxide were detected for 8 days, and therefore, they could be monitored alongside the parent drug as evidence that the substance has gone through the horse's body.