1. The developmental expression of an inwardly rectifying current activated by membrane hyperpolarization (Ih) and of a tetrodotoxin (TTX)-resistant Na+ current (INa(TR)) was studied using freshly dissociated ganglionic quail neurones of various embryonic ages. This work was carried out on parasympathetic (ciliary) and sensory (trigeminal and dorsal root) ganglion neurones with the whole-cell configuration of the patch-clamp technique. 2. In sensory and parasympathetic neurones, Ih was activated at potentials more negative than -60 mV and displayed strong inward rectification. No sign of time- or voltage-dependent inactivation was apparent. Ih was carried by both Na+ and K+ ions and was selectively and reversibly blocked by extracellular Cs+. 3. During the development of sensory neurones, Ih was observed for the first time between embryonic day 10 (E10) and E11 and the percentage of neurones expressing the current increased subsequently, reaching a plateau level of about 80% at E14. In the parasympathetic neurones of the ciliary ganglion, Ih was already detected at E10 and the percentage of neurones possessing the current increased until E16, a stage at which all neurones were found to express Ih. 4. In the presence of TTX (1 microM), an inward Na+ current, INa(TR), was recorded in sensory neurones after E12. This current was activated at potentials more depolarized than -30 mV and its amplitude was maximal at +5 mV. INa(TR) showed time- and voltage-dependent inactivation. Half-maximal steady-state inactivation was observed at -40 mV. 5. INa(TR) was observed for the first time after E12 in sensory neurones and the percentage of neurones with INa(TR) increased until E14. Thereafter, 80% of the neurones had the current. In contrast, INa(TR) was never observed in the parasympathetic neurones of the ciliary ganglion during embryonic development. 6. Our results with parasympathetic and sensory neurones suggest that the expression of INa(TR) is linked to the phenotype and not to the embryonic origin of a neurone.