1. Responses induced by intracellular cyclic nucleotides were analysed in isolated olfactory receptor cells of the newt under a voltage-clamp condition by using the patch pipette in a whole-cell recording configuration. Cyclic nucleotides were applied by diffusion from the patch pipette. 2. Introduction of either cyclic AMP or cyclic GMP caused a transient inward current in cells held at -50 mV. The response amplitude was dose-dependent with the Hill coefficient of 3 and half-saturating concentration of 300 microM (concentration in the pipette) for both cyclic AMP and cyclic GMP. Cyclic CMP was less effective than those two nucleotides. 3. The response to intracellular cyclic AMP was seen in all cilia-bearing cells, but not in cells which lost the cilia during dissociation. The response latency was shorter when cyclic AMP was introduced into the ciliated terminal swelling (ca 0.2 s) rather than into the cell body (ca 1.4 s). These results suggest that the sensitivity to intracellular cyclic AMP is confined to the cilia. 4. The cyclic AMP-induced current was transient (half decay time, ca 2.3s) despite the fact that cyclic AMP was continuously loaded from the patch pipette. The response time course was controlled by Ca2+; the reduction of external Ca2+ concentration (replaced with Mg2+) or loading the cell with 50 mM-EGTA prolonged the cyclic AMP-induced responses. The Ca2(+)-induced suppression was reversible. 5. The reversal potential of the cyclic AMP-induced transient current was -4.8 +/- 3.8 mV, and that of the current re-induced by Ca2+ removal was 1.5 +/- 2.1 mV, suggesting that both currents flowed through the same ionic channel. The channel permeates all alkali metal ions with the permeability ratios of PLi:PNa:PK:PRb:PCs = 0.93:1:0.93:0.91:0.72, but not Cl- or choline ions. 6. These results demonstrate that the cyclic AMP-induced response and the odorant-induced response of the isolated olfactory cell have nearly identical characteristics. The present study supports the notion that cyclic AMP is the internal messenger mediating olfactory transduction.