1. The distribution of pharmacologically and/or biophysically unique Ca2+ current subtypes was studied in different diameter rat dorsal root ganglion (DRG) neuron cell bodies. DRG cells which fell into three diameter ranges, small (20-27 microns), medium (33-38 microns) and large (45-51 microns), were studied. T-type Ca2+ current was defined as low-threshold, rapidly inactivating current evoked by a weak test depolarization (-50 mV) from negative holding potentials (-80 to -100 mV), and which was sensitive to changes in holding potential. L-type Ca2+ current was defined as peak high-threshold Ca2+ current evoked from a holding potential of -60 mV and sensitive to blockade by 2 microM-nimodipine. N-type Ca2+ current was defined as peak high-threshold Ca2+ current evoked from a holding potential of -60 mV and sensitive to blockade by 0.9 microM-omega-conotoxin GVIA. 2. T-type Ca2+ currents were observed in small and medium diameter, but not in large diameter, DRG cell bodies. Large diameter DRG cell bodies had a small amount of low-threshold Ca2+ current but this current did not inactivate and was insensitive to a change in holding potential from -80 to -90 mV, and thus did not appear to be conducted through T-type Ca2+ channels. The T-type Ca2+ currents observed in medium diameter DRG cell bodies were considerably larger in amplitude (1-6 nA) than those observed in small diameter DRG cell bodies (100 pA-1 nA). This difference could not be accounted for by the difference in membrane surface area of small versus medium diameter DRG cell bodies. 3. The T-type Ca2+ currents observed in medium diameter DRG cells were sensitive to blockade by amiloride. Amiloride (500 microM) blocked 79.4 +/- 0.9% (mean +/- S.E.M.) of T-type Ca2+ current amplitude in six medium diameter DRG cell bodies which were held at -80 mV and depolarized to -50 or -40 mV. Amiloride (500 microM) failed to block high-threshold current in five medium diameter DRG cell bodies, indicating that it was specific for T-type Ca2+ current in these cells. 4. The percentage of peak whole-cell L-type Ca2+ current was significantly larger in small diameter DRG cell bodies (52.9 +/- 4.7% of total whole-cell Ca2+ current) than in medium diameter DRG cell bodies (6.6 +/- 3.9% of total whole-cell Ca2+ current) or large diameter DRG cell bodies (19.4 +/- 5.7% of total whole-cell Ca2+ current).(ABSTRACT TRUNCATED AT 400 WORDS)