Following intravenous (i.v.) or intramuscular (i.m.) adminstration of sulphadimidine (SDM), the pharmacokinetics of SDM and N4‐acetylsulphadimidine (N4‐SDM) were studied in plasma of calves from the first day of life to the age of about 6 months. An obvious age dependency was observed for the elimination half‐life (t1/2) of SDM: the first day of life the t1/2 ranged between 13.5 and 17 h, and decreased in approximately 3 weeks to 4–6 h and remained constant from this time. The metabolite N4‐SDM, as a percentage of the total concentration of the sulphonamide measured in plasma of neonatal calves, ranged between 21.6 and 25.5% at the first day of life, declined in 3 weeks to approximately 12.8%, and at 5 till 9 weeks the final percentage was about 6.8%. Following administration of N4‐SDM, the elimination half‐life of N4‐SDM was 3 h in an 8‐day‐old calf declined to 1.4–1.7 h in 4‐week‐old calves, and was 0.9 h in calves older than 11 weeks. The percentage of SDM (a metabolite of N4‐SDM) in plasma increased with time after injection from 5.5 to 62.8% of the total sulphonamide plasma concentration. This value was age‐related. The total body clearance of N4‐SDM was three‐ to