The permeability of biological membranes to boric acid was investigated using the giant internodal cells of the charophyte alga Chara corallina (Klein ex Will. Esk. R.D. Wood). The advantage of this system is that it is possible to distinguish between membrane transport of boron (B) and complexing of B by plant cell walls. Influx of B was found to be rapid, with equilibrium between the intracellular and extracellular phases being established after approximately 24 h when the external concentration was 50 microM. The intracellular concentration at equilibrium was 55 microM, which is consistent with passive distribution of B across the membrane along with a small amount of internal complexation. Efflux of B occurred with a similar half-time to influx, approximately 3 h, which indicates that the intracellular B was not tightly complexed. The concentration dependence of short-term influx measured with 10B-enriched boric acid was biphasic. This was tentatively attributed to the operation of two separate transport systems, a facilitated system that saturates at 5 microM, and a linear component due to simple diffusion of B through the membrane. Vmax and Km for the facilitated transport system were 135 pmol m(-2) s(-1) and 2 microM, respectively. The permeability coefficient for boric acid in the Chara plasmalemma estimated from the slope of the linear influx component was 4.4 x 10(-7) cm s(-1) which is an order of magnitude lower than computed from the ether:water partition coefficient for B.