A large-conductance calcium channel in chicken dorsal root ganglion neurons was studied with patch-clamp recordings of unitary currents. In addition to the conventional pattern of Ca-channel gating previously described in neurons ("mode 1"), we observed a different form of gating behavior ("mode 2"). Unlike the brief (approximately equal to 1 ms) openings in mode 1, mode 2 openings tend to be longer (greater than 10 ms) and often outlast the test pulse. In mode 2, the probability of channel openness (P) is high at relatively negative potentials where P in mode 1 is low. Mode 2 activity appears much less often than mode 1 activity in the absence of drug. However, the balance is strongly shifted in favor of mode 2 by the dihydropyridine Ca agonist Bay K 8644, an effect that underlies a marked enhancement of Ca-channel activity. This is the first evidence for dihydropyridine control of neuronal Ca-channel function at the single-channel level. Sweeps showing mode 1 or mode 2 gating appeared interspersed with sweeps with no openings, during which the channel was unavailable for opening ("null mode" or "mode 0"). Two approaches showed that switching between all three modes occurred on a time scale of seconds: (i) channels tended to remain in the same mode from one sweep to the next, with pulses at 0.25 Hz; and (ii) steady depolarizations in Bay K 8644 produced clusters of mode 2 openings lasting several seconds. Changes in the rates of switching might be important in neurochemical modulation of Ca channels. Bay K 8644 and other dihydropyridine Ca agonists might be useful experimental tools for manipulating transmitter release, neurite extension, and other neuronal functions dependent on intracellular Ca.