1. Whole-cell patch-clamp recording was performed from principal neurones of the substantia nigra pars compacta (SNc). In 66% of these neurones, neurotensin (NT) induced, at -60 mV, an inward current associated with an increase in conductance. 2. Principal neurones displayed, in response to hyperpolarizing voltage steps, the voltage-dependent inward cationic current, Ih. This current activated at potentials more negative than -65 mV and reached a maximum at -106 +/- 4 mV, with a half-activation potential of -86 +/- 3 mV. Its estimated reversal potential was -43 +/- 7 mV and its activation curve was fitted with two exponentials. 3. In 41% of neurones showing the inward current, NT (0.5 microM) also reversibly reduced the amplitude of Ih. The diminution was 48.5 +/- 12% when voltage steps were made from -60 to -95 mV. The decrease in Ih resulted from a reduction in the maximal current with no change in the voltage dependence of activation. 4. Forskolin (10 microM), an activator of adenylate cyclase, increased Ih by shifting its activation range to more positive potentials, but it did not alter the NT inhibition of Ih. 5. The effect of NT was blocked by staurosporine (0.5 microM) and by PKC-(19-31) (0.5 microM), a specific protein kinase C (PKC) inhibitor, but was unaffected by Walsh's peptide (100 microM), a specific inhibitor of protein kinase A. The reduction of Ih was mimicked by 1-oleoyl-2-acetyl-sn-glycerol (0.5-10 microM), an analogue of diacylglycerol, an endogenous PKC activator. 6. These results suggest that the inhibition of Ih by NT involves a phosphorylation mechanism that implies activation of PKC.