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)