1. Calcium channel activity was recorded in chromaffin cells in the cell-attached condition, using 110 mM-Ba2+ as the permeant ion. 2. One type of calcium channel had a conductance of 16 pS, was completely inactivated at a holding potential of -20 mV and was insensitive to dihydropyridine agonists and antagonists. These characteristics correspond to a calcium channel of the N-type. 3. A second type of calcium channel was active at holding potentials of -30 mV and above, had a channel conductance of 31 pS, and was sensitive to the dihydropyridine agonist, Bay K 8644. The channel opened along two dominant modes with characteristic time constants of 0.5 and 5 ms. The main effect of Bay K 8644 was to increase the probability of both short and long openings with no change in their relative proportions (6 to 1 respectively). These characteristics correspond to a calcium channel of the L-type. 4. omega-Conotoxin affected the activity of both N- and L-type channels. It drastically reduced the number of N-type channel openings and produced complex changes in L-type channel activity. Long openings were less frequent and the conductance during short openings was slightly smaller than that measured in the presence of Bay K 8644. 5. The discussion focuses on modulation of L-type channel activity. Openings of L-type channels are rarely recorded in the cell-attached configuration under control conditions. Addition of Bay K 8644 is needed to reveal the presence of L-type channels. By contrast, L-type currents recorded in the whole-cell configuration are always observed and are insensitive to Bay K 8644. These results indicate that L-type channels are normally inoperable in chromaffin cells.