1. We investigated the effects of histamine on membrane currents and contractile state of isolated guinea-pig tracheal myocytes using perforated patch and whole-cell recording techniques. The effects of histamine were compared to those of acetylcholine (ACh) and caffeine. 2. During voltage clamp (Vhold = -60 mV), histamine elicited contraction and an inward current (Ihist) which was often followed by current oscillations. Ihist had a reversal potential (Vrev) of -9 +/- 3 mV. 3. Ihist was dependent on the Cl- gradient and was antagonized by the Cl- channel blocker niflumic acid. Vrev was more positive (+2 +/- 1 mV) when K(+)-selective currents were blocked by Cs+ and TEA. When all external Na+ was replaced with N-methyl-D-glucamine, there was a small reduction in the amplitude of Ihist. 4. The histamine-induced current was similar to that elicited by ACh and by caffeine with respect to time course, amplitude, and current-voltage relationship. Responses to histamine and to ACh were non-additive, consistent with a convergence of histaminergic and cholinergic signalling pathways. Ihist was antagonized by the H1 histaminergic receptor antagonist astemizole, but not by atropine. 5. When recorded using the perforated patch configuration, Ihist could be elicited repeatedly for more than 30 min. When cells were studied in the whole-cell configuration using a pipette solution containing 0.025 mM EGTA, the amplitude of Ihist was initially the same as that obtained using perforated patch but then decreased; the time required for the responses to decrease to 50% (t1/2) was 8.2 +/- 1.0 min. When 1 mM EGTA was included in the pipette solution (whole-cell configuration), the initial response to histamine was significantly decreased in size and t1/2 was reduced to 3.3 +/- 0.7 min. 6. The characteristics of the signalling pathway were examined in cells studied using the whole-cell configuration with 0.025 mM EGTA in the recording pipette. Heparin significantly reduced t1/2 to 4.3 +/- 0.8 min. GTP gamma S elicited inward current and oscillations; both effects were enhanced by histamine. GTP gamma S also reduced t1/2 to 1.4 +/- 0.1 min. Pertussis toxin did not alter the amplitude or time course of Ihist. 7. We conclude that in guinea-pig tracheal myocytes, binding of histamine to H1 receptors leads to release of Ca2+ from intracellular stores and subsequent activation of Cl- and K+ conductances as well as contraction. Furthermore, we demonstrate that ACh elicits similar physiological responses due to a convergence of the histaminergic and muscarinic signalling pathways.