Intestinal transport of baclofen (β-(p-chlorophenyl)-γ-aminobutyric acid) in the rat has been examined in vitro. Influx of baclofen across the brush-border membrane (JmcBacl) and steady-state accumulation by everted segments of the intestine were measured. JmcBacl could be accounted for as the sum of a saturable process with a maximum rate of approx. 10-20 nmol cm-2 h-1 and a K 1 2Bacl of approx. 0.3 mM and a diffusive contribution with a permeability of 0.073 cm h-1. JmcBacl was Na+- and Cl--independent. The steady state distribution ratio between the intracellular space of the everted segments and incubation fluid was 1.0 ± 0.1 (n = 12). Inhibition tests demostrated that the Na+- and Cl--independent, passive, but saturable fraction of baclofen transport can not be mediated by any of the known amino acid carriers of the rat small intestine. Preliminary results suggests that qualitatively baclofen transport in guinea-pig and rabbit is also by facilitated transport.