1. The actions of 5-hydroxytryptamine (5-HT) and some 5-HT1A receptor ligands on neurones in the rat dorso-lateral septal nucleus were recorded in vitro by intracellular recording techniques. 2. In the presence of tetrodotoxin (1 microM) to block any indirect effects, bath application of 5-HT (0.3-30 microM) hyperpolarized the neurones in a concentration-dependent manner and reduced membrane resistance. The hyperpolarization did not exhibit desensitization and was sometimes followed by a small depolarization. 3. The 5-HT1A receptor ligands, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), N,N-dipropyl-5-carboxamidotryptamine (DP-5-CT) and buspirone but not the non-selective 5-HT1 receptor agonist, 1-m-trifluoromethylphenylpiperazine (TFMPP), also hyperpolarized the neurones. 4. 5-HT, 8-OH-DPAT and DP-5-CT appeared to act as full agonists whereas buspirone behaved as a partial agonist. The estimated EC50S were: DP-5-CT 15 nM, 8-OH-DPAT 110 nM, 5-HT 3 microM and buspirone 110 nM. 5. At a concentration of 3 microM, the putative 5-HT1A receptor antagonists, spiperone, methiothepin, NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-pthalimido)butyl]piperazine) and MDL 73005EF (8-[2-(2,3-dihydro-1,4-benzodioxin-2-yl-methylamino)ethyl]-8- azaspiro[4,5]decane-7,9-dione methyl sulphonate), produced a parallel rightward shift in the concentration-response curve to 5-HT with no significant reduction in the maximum response. The estimated pA2 values were: NAN-190 6.79, MDL 73005EF 6.59, spiperone 6.54 and methiothepin 6.17.6. The 5-HT2/5-HTlc receptor antagonist, ketanserin (3 microM) and the 5HT3 receptor antagonist, tropisetron (3 microM) did not antagonize the 5-HT-induced hyperpolarizations; however, ketanserin blocked the depolarization which sometimes followed the hyperpolarization.7. It is concluded that the 5-HT-induced membrane hyperpolarization of rat dorso-lateral septal neurones is mediated by 5-HTA receptors.