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The pharmacokinetic behaviour of enrofloxacin was compared in four one-day-old and four one-week-old calves in order to find out if there were any age-related differences. Mean volume of distribution (Vd(ss)) and clearance (Cl) were significantly smaller in newborn calves: Vd(ss) was 1.8 and 2.3 L/kg, while clearance was 0.19 and 0.39 L/kg.h in newborn and one-week-old calves, respectively. Mean elimination half-life (t1/2β) did not differ significantly in newborn and in one-week-old calves: mean t1/2β was 6.6 h and 4.9 h, respectively. Enrofloxacin was metabolized to ciprofloxacin both by newborn and one-week-old calves, but the maximum concentration (Cmax) of ciprofloxacin was lower and the time when maximum concentration was reached (tmax) later in newborn calves. We conclude that the dosage of enrofloxacin should be adjusted according to age when administered to very young calves.