The half-lives, apparent volume of distribution and protein-binding of 11 sulphonamides were determined in 89 experiments on 49 cows. The estimations of half-lives indicated the presence of a distribution phase (alpha-phase) for all the sulphonamides investigated with the exception of sulphachloropyridazine. The elimination half-life (beta-phase) of the sulphonamides in plasma varied from 70 to 1000 min and was positively correlated with the solubility of the compounds in organic solvents. This was explained by the greater reabsorption of the more fat-soluble compounds in the kidneys. All the sulphonamides except sulphadimidine had a shorter half-life in cows than previously reported from human investigations. The apparent volume of distribution was about one for sulphanilamide and lower for all the other sulphonamides investigated. The protein-binding estimated in vitro agreed well with the in vivo results. It was slightly lower than in humans and the degree of protein-binding decreased with increasing sulphonamide concentration in plasma.