La exchange was studied in guinea pig taenia coli in high K medium. Tissue content of La, as well as of Na, K, and CI, was determined by thermal neutron activation analysis, after which the tissue was subjected to atomic absorption photometry to determine Ca content. With all La concentrations (0.125-5.00 mM), time course of La uptake by tissue(excluding extracellular fluid) showed two components: one(CI) was saturated after 15 min contact with La, and another(C2) was not even after 180 min contact. Washout experiments indicated that each component consists of two fractions: exchange able(e) and inexchangeable(i). In each fraction, which can be multicompartmental, overall exchange rates and capacity were estimated as shown in the table. Mathematical analysis by general transfer equation and by others based on simple assumptions was not applicable to each of the above four fractions. Relation of La to the other ions and to mechano-grams will be discussed elsewhere.