The pharmacokinetic parameters of erythromycin in foals were determined following intravenous administration of 5.0 mg/kg to animals aged 1, 3, 5 and 7 weeks. The distribution of the drug was described by a two-compartment open model, and no significant differences were observed between coefficients on which the parameters were based. Pharmacokinetic values were also determined for four mares given 5.0 mg/kg intravenously and for six 10-12-week-old foals given 20.0 mg/kg intravenously. The half-life of erythromycin for all groups of animals (foals less than 7 weeks, mares, foals 10-12 weeks) was 1.0-1.1 h; the apparent volume of distribution was between 2.3 and 7.2 l/kg, and the clearance of the drug from the body was between 1.9 and 5.0 mg/kg/h. No drug could be detected in the serum following oral administration of 5.0 mg/kg erythromycin estolate; detectable levels were found for 5 h in mares given 12.5 mg/kg, and for 8 h in foals given 20.0 mg/kg orally. Peak levels in foals given the drug orally were 0.42 micrograms/ml at 120 min after administration. Foals given 10.0 mg/kg of erythromycin base intramuscularly had serum concentrations detectable 12 h later; the peak level achieved was 1.44 micrograms/ml serum 90 min after administration and concentrations exceeded 0.25 micrograms/ml for 6 h. In the mares the milk concentrations were approximately twice those in serum. Recommendations were made for drug dosage to be used in the treatment of Corynebacterium equi pneumonia of foals.