The 'high affinity' uptake of GABA was studied in slices of rat cerebral cortex using the procedure developed by Iversen and Neal5. The high affinity uptake of L-glu-tamate, glycine, and L-proline was studied in a similar manner6'8. Arecaidine was prepared from commercially available arecoline by acidic hydrolysis9. Guvacine was prepared as described by McElvain and Stork10, and guva-coline was prepared from this by esterification according to Freudenberg11. The four compounds were analytically pure, and their infrared and proton magnetic resonance spectra were consistent with the structures in Fig. 1. Fig. 1 Structures of nipecotic acid, guvacine, guvacoline, arecaidine and arecoline.Arecoline and guvacoline (0.5 mM) had no effect on the uptake of GABA or L-glutamate. Arecaidine and guvacine (0.5 mM) inhibited the uptake of GABA by 753 and 98 + 1% (mean + s.e.), respectively, but did not inhibit the uptake of L-glutamate, glycine or L-proline. Probit analysis of the results of experiments with appropriate concentrations of arecaidine and guvacine indicated that 50% inhibtion of GABA uptake was obtained with 12212/M arecaidine and 81 /M guvacine. On this basis, guvacine was as potent as nipecotic acid3, which inhibited GABA uptake by 50% at 9l nM. All of these experiments were carried out with pre-incubation of the inhibitor with the brain slices for 15 min before addition of radioactive GABA. As has been observed with nipecotic acid12 and 2,4-diaminobutyric acid13, less apparent inhibition of GABA uptake was obtained if arecaidine or guvacine were added at the same time as GABA. This may be interpreted on the basis of these inhibitors being substrates for the GABA uptake system and entering intracellular compartments during the pre-incubation period. Kinetic studies showed that arecaidine and guvacine act as competitive inhibitors of GABA uptake when added at the same time as GABA. The apparent slope inhibition constants (K\s) were 141+38/iM for arecaidine and 14 + 4/iM for guvacine. On this basis, guvacine has a similar affinity for the GABA carrier to nipecotic acid3 which has a K\s of 113 j^M. These experiments are consistent with arecaidine and guvacine acting in rat brain slices as substrate-competitive inhibitors of GABA uptake, binding to the GABA carrier and penetrating the tissue. Such an action in vivo could contribute to the observed effects of arecaidine administered to mice, which includes reduction in spontaneous activity, exploration and motility2. Antagonism of the inactivation of GABA might lead to prolongation of GABA-induced synaptic inhibition, such as that proposed for pentobarbitone, which at 5 mM inhibits (58%) the uptake and potentiates (46%) the release of GABA in rat brain slices14. Arecaidine, injected subcutaneously into mice in doses of 50-100 mg kg"1 (0.3-0.6 mM if evenly distributed in body fluids), has been shown to prolong barbiturate narcosis and to inhibit spontaneous activity2. We have shown that guvacine, injected intraperitoneally into mice in doses of 50-100 mg kg"1 provoked a pronounced but rather short lived reduction of spontaneous activity, the duration of action being 15-30 min.Guvacine is more potent than arecaidine with respect to inhibtion of GABA uptake in vitro. The above mentioned results may stimulate further studies on the pharmacology of guvacine and arecaidine, and preparation of structural analogues of these amino acids as potential psychoactive agents. We thank the Danish MRC for support.