SUMMARY: Freshly-dispersed bovine trachealis cells were used for recording by the patch clamp technique of whole-cell and unitary currents through (\mathrm{Ca}^{2+})-channels. Whole-cell (\mathrm{Ca}^{2+})-current (\left(\mathrm{I}{\mathrm{Ca}}\right.) ) activated at (-\mathbf{4 0 \mathrm { mV }}) and appeared to be carried by a single type of (\mathbf{C a}^{2+})-channel. Inactivation of (\mathrm{I}{\mathrm{Ca}}) was increased by increasing the concentration of free (\mathrm{Ca}^{2+}) within the recording pipette but reduced by using (\mathrm{Ba}^{2+}) as the charge carrier. Steady-state inactivation studies showed that the (\mathrm{Ca}^{2+})-channels were half-maximally available following a conditioning depolarization to (-35 \mathrm{mV}). A two-pulse protocol showed that (I_{\mathrm{Ca}}) induced by the step to a test potential was inversely related to (\mathrm{I}{\mathrm{Ca}}) induced by the step to the conditioning potential. Unitary (\mathbf{B a}^{2+})-currents were activated at a threshold of (-30 \mathrm{mV}) and had a reversal potential of (+41 \mathrm{mV}). The channel carrying the unitary (\mathrm{Ba}^{2+})-currents had a slope conductance of (23 \mathrm{pS}). Steady-state inactivation studies showed that the unitary (\mathrm{Ba}^{2+})-currents were half-maximally available at a holding potential of (-28 \mathrm{mV}). (\mathrm{I}{\mathrm{Ca}}) and unitary (\mathrm{Ba}^{2+})-currents were inhibited by nifedipine ( (10 \mathrm{nM}-1 \mu \mathrm{M}) ) but augmented by Bay (\mathrm{K} 8644(10 \mu \mathrm{M})). It is concluded that the plasmalemma of bovine trachealis muscle contains a single population of voltage-dependent (\mathbf{C a}^{2+})-channels of the (\mathbf{L})-type. These channels may be subject to inactivation primarily by an increase in the concentration of free (\mathrm{Ca}^{2+}) on the cytosolic side of the plasmalemma and secondarily by a voltage-dependent mechanism. Overlap of the inactivation and activation curves of (\mathrm{I}_{\mathrm{Ca}}) may allow the passage of 'window current' through the (\mathbf{C a}^{2+})-channels during sustained depolarization.