Nicotinic acetylcholine receptors present on cultured hippocampal neurons from fetal rats were characterized by means of whole-cell patch-clamp technique, using a number of structurally divergent agonists and highly selective antagonists. Based upon the decay kinetics of the currents elicited by 3 mM acetylcholine (ACh) and their sensitivities to agonists and antagonists, the neurons were shown to exhibit four current types, IA, IB, II and III. Rapidly decaying currents (type IA) that were blocked by alpha-bungarotoxin (10 nM), kappa-bungarotoxin (10 nM) and methyllycaconitine (MLA, 1 nM) were the most frequent and were found in 83% of the neurons tested. Type II currents (found in 5% of the neurons) were blocked by dihydro-beta-erythroidine (10 nM), and by high concentrations of MLA and kappa-bungarotoxin (100 nM each) but not by alpha-bungarotoxin (100 nM). Type III currents (elicited in 2% of the neurons) decayed slowly and were blocked by (+/-)-mecamylamine (1 microM) but not by alpha-bungarotoxin, kappa-bungarotoxin or MLA (each at 100 nM). Some of the cells (10% of the neurons) had mixed responses (named type IB), which were only partially blocked by MLA (1 nM) or dihydro-beta-erythroidine (10 nM) alone and were completely blocked by combination of the two agents. The order of potency of agonists in activating the currents was the following: for type IA, (+)-anatoxin-a >> 1,1-dimethyl-4-phenyl-piperazinium > (-)-nicotine > cystisine > ACh > carbachol > (+)-nicotine > arecoline > suberyldicholine; for type II, ACh > (+)-anatoxin-a > (-)-nicotine > 1,1-dimethyl-4-phenyl-piperazinium > carbachol > cytisine > (+)-nicotine > suberyldicholine > arecoline. Certain agonists were particularly useful in discriminating among the various types of currents: ACh, carbachol, (-)-nicotine and suberyldicholine for type II, and cytisine for type III currents. The EC50 of ACh was approximately 130 microM for type IA and approximately 2 microM for type II currents. A marked inward rectification was observed with type II, whereas type IA currents showed very little inward rectification. Differences observed in the pharmacological and functional properties of the nicotinic currents imply the expression of at least three structurally distinct nicotinic acetylcholine receptor subtypes in hippocampal neurons. The possible involvement of these currents in the transduction of signals is discussed.