1. Normal activity in bilateral pairs of heart interneurons, from ganglia 3 or 4, in the medicinal leech (Hirudo medicinalis) is antiphasic due to their reciprocally inhibitory connections. However, Ca(++)-free Co(++)-containing salines lead to synchronous oscillations in these neurons. 2. Internal TEA+ allows expression of full plateaus during Co++ induced oscillations in heart interneurons; these plateaus are not blocked by Cs+. Similar plateaus are also observed with internal TEA+ alone, but under these conditions activity in heart interneurons from ganglia 3 or 4 is antiphasic. 3. Plateaus in heart interneurons induced by Co++ and internal TEA+ involve a conductance increase. 4. A voltage-dependent inward current, IP, showing little inactivation, was isolated using single-electrode voltage-clamp in heart interneurons. This current is carried at least in part by Na+; the current is reduced when external Na+ is reduced and is carried by Li++ when substituted for Na+. 5. Calcium channel blockers such as La3+ and Co++ block neither the TEA+ induced plateaus nor IP, suggesting that Na+ is not using Ca++ channels. Moreover, IP is enhanced by Ca(++)-free CO(++)-containing salines. Thus, IP is correlated with the TEA(+)- and Co(++)-induced plateau behavior.