Experiments were carried out in pigs to ascertain to what extent the cation exchangers Ujolyt® and Campanyl® used in the prevention of urinary stones undergo persorption and appear in the urine. We used two preparations of different grain size and detected them by labelling with 35S. A maximum of 0.5% or 5 × 10––3 of the dose was found as persorbed particles at 51 h, chiefly in the muscles. The number of persorbed particles from the fine-grained preparation was considerably greater than that from the coarse-grained product. The urine contained the smallest proportion of particles, less than 2 × 10––5 of the dose. In view of these results there is no reason to believe that solid particles persorbed during treatment with ion exchange resins can act as crystallization centres for stone formation, and it is equally unlikely that ion exchangers have to reach the urine before they can exert their effect.