Excretion of Some Drugs in Bovine Tears

The excretion of drugs in the tears of cattle was studied, while almost constant plasma concentrations were maintained by continuous intravenous infusion. The examination included both acid and basic drugs. A fairly constant ratio between the concentrations in the tears and the ultrafiltrate of plasma was found for each drug examined. The ratio was not influenced by the rate of secretion or the plasma concentration. For drugs of an acid nature the ratio was less than or equal to 1 (sulphanilamide 0.99, sulphadimidine 0.80, sulphadiazine 0.35, sulphadoxine 0.48, benzylpenicillin 0.004), whereas the ratio for drugs of a basic nature was greater than 1 (penethamate 7.1, trimethoprim 2.2). The above described investigations provide reasons for believing that the passage of the examined drugs from the plasma to the tears may take place by diffusion of the non‐protein‐bound, unionized fraction.