Drug concentrations in serum, synovial fluid and tissue cage fluid (TCF) in calves were measured after single i.m. doses of oxytetracycline hydrochloride (OTC), procaine penicillin G (PPG) and potassium penicillin G (KPG) and single i.v. doses of sulphadimidine (SDM) and OTC. For all drugs, concentration—time curves in serum and synovial fluid were not identical but they had similar profiles, with peak levels occurring at about the same time. Concurrent concentrations were lower in synovial fluid than in serum. For each drug, elimination half‐lives from synovial fluid and from serum were similar, except for penicillin G after KPG administration which had a significantly longer half‐life from synovial fluid than from serum (P<0.05). Of the two penicillin G preparations, PPG gave a signficantly higher synovial fluidrserum area under curve (AUC) ratio than did KPG; 0.76 ± 0.10 and 0.54 ±0.12, respectively (P0.05). Potassium penicillin G (KPG), however, gave a significantly higher TCF:serum Af/C‐ratio than PPG; 0.55 ± 0.21 and 0.19 ± 0.07, respectively (P<0.05). It is concluded that serum concentrations of these antibacterial drugs give an approximate indication of levels in synovial fluid, whereas concentrations in TCF are considerably lower, with a slow rise and decline characteristic