The linkage between cation and Cl transport in the form of Na-Cl or Na-K-Cl cotransporters is a widespread phenomenon in animal cells (Haas and McManus, 1983; Palfrey and Rao, 1983; Tas et al., 1987). Scant attention, however, has been paid to the mammalian choroid plexus (CP) in regard to the function, or even the existence, of cation-Cl cotransport. Pharmacologic investigations of the CP-CSF system of intact animals have furnished evidence for cation-Cl cotransport in some experiments (McCarthy and Reed, 1974; Melby et al., 1982), but not in others (Vogh and Langham, 1981; Miller et al., 1986). We have recently used an isolated preparation to analyse ion transport in CP (Parmelee and Johanson, 1989). Such an in vitro analysis permits fine control of artificial CSF concentrations of ions and drugs over a wide range (Bairamian et al., 1990; Johanson et al., 1990). This approach has enabled us to obtain solid evidence for Na-K-Cl cotransport in rat CP.