1. The membrane properties and conduction velocities of antidromically activated medial septum-diagonal band (MS-DB) neurons were examined using whole-cell recordings in a longitudinally cut rat brain slice preparation containing the MS-DB and the dorsal fornix. 2. MS-DB neurons were divided into three groups according to their action potential characteristics and firing properties. Slow firing neurons displayed a broad action potential followed by a prominent after-hyperpolarization. Burst firing neurons, when depolarized from hyperpolarized holding potentials, exhibited a high-frequency burst of spikes on the crest of a slow depolarizing potential. Fast firing neurons did not fire bursts of spikes when depolarized from hyperpolarized holding potentials. 3. Eighteen MS-DB neurons were identified as septohippocampal by antidromic activation. Of the septohippocampal neurons, four were slow firing neurons, five were burst firing neurons and nine were fast firing neurons. The mean axon conduction velocities of these neurons fell into two significant groups, termed slow conducting and fast conducting. Slow firing septohippocampal neurons had significantly slower conduction velocities than either fast firing or burst firing neurons (P < 0.05), being 0.7 +/- 0.5 ms-1 for slow firing neurons and 2.9 +/- 2.0 and 2.0 +/- 1.4 ms-1 for burst firing and fast firing neurons, respectively. 4. On the basis of previous evidence which has linked firing properties with the neurochemical identities of the neurons, we propose that the slow firing septohippocampal neurons are cholinergic whereas the burst firing and fast firing septohippocampal neurons are GABAergic.