In this review the pharmacokinetic properties of trimethoprim (TMP) and TMP/sulphonamide combinations are discussed. The concentration of both substances in the various body fluids and tissues differ considerably from the concentrations in blood due to the different distribution of TMP and sulphonamides in the body. As a rule the ratio of the concentration of TMP to sulphonamide is higher in most body fluids and secretions than in blood. If inflammation is present, the concentrations vary considerably. The pharmacokinetic data must be taken into account when making in vitro tests. At the site of inflammation a TMP/sulphonamide concentration factor of 1:0.5 to 1:4 can be expected in urine and of 1:2 to 1:5 in tissue.