1. The membrane properties and synaptic inputs of interneurons, located at the stratum (s.) lacunosum-moleculare and radiatum border (L-M) of the CA1 region, were examined with the use of current-clamp whole-cell recordings in rat hippocampal slices. 2. Biocytin-labeled L-M interneurons had nonpyramidal somata and aspinous, often beaded, dendrites that arborized in s. lacunosum-moleculare and radiatum, sometimes as far as s. moleculare of the dentate gyrus. Their axon coursed and branched in s. lacunosum-moleculare and radiatum. Axon collaterals were also observed traversing the hippocampal fissure and arborizing in s. moleculare of the dentate gyrus and s. radiatum of the CA3 region. 3. Several membrane properties of interneurons were typically nonpyramidal: they had large input resistances, short-duration action potentials followed by prominent fast afterhyperpolarizations, and responded to hyperpolarizing current pulses with little membrane rectification. L-M interneurons showed significant anodal break responses, and their mean membrane time constant was 33 ms. After-depolarizations elicited by subthreshold depolarizing current pulses were larger in amplitude and decayed more slowly at depolarized than hyperpolarized membrane potentials. 4. The majority of L-M interneurons (35 of 49 cells) were silent at resting membrane potentials, whereas other displayed either spontaneous single action potentials (n = 12) or rhythmic bursts (n = 2). The rhythmic bursts were insensitive to the N-methyl-D-aspartate (NMDA) and non-NMDA excitatory amino acid receptor antagonists, 2-amino-5-phosphonopentanoic acid (AP-5; 50 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20 microM), respectively. Both spontaneous single action potentials and burst firing were blocked by membrane hyperpolarization, suggesting that they were intrinsically rather than synaptically generated. 5. L-M interneurons responded with regular sustained firing to depolarizing current pulses at resting membrane potential. However, at more hyperpolarized membrane potentials (near -75 mV), depolarizing current pulses elicited action-potential firing with a delayed onset. This suggests that voltage-sensitive, transient outward currents may be activated in L-M interneurons from hyperpolarized membrane potentials. 6. Electrical stimulation of s. radiatum or lacunosum-moleculare elicited predominantly long-duration excitatory postsynaptic potentials (EPSPs; n = 20 cells), or both EPSPs and inhibitory postsynaptic potentials (IPSPs; n = 17 cells). In most L-M interneurons (35/37), with increasing intensities, up to two action potentials were elicited. Occasionally, larger bursts (3-5 action potentials) were observed (n = 2). 7. The multiphasic components of the synaptic responses became more evident when stimulations were repeated at different membrane potentials.(ABSTRACT TRUNCATED AT 400 WORDS)