Single-channel activities of homotetrameric recombinant rat type 1 InsP3R channels expressed in mutant DT40-3KO cells with no endogenous InsP3R expression were studied in nuclear patch clamp experiments. In on-nucleus experiments with steady ligand conditions, the channel exhibits modal gating with at least two modes: a high-Po mode with Po = 0.4-0.6 and mean open duration (to) ∼ 3 ms; and a low-Po mode with Po < 0.05 but to still ∼ 3 ms. Cytoplasmic free Ca2+ concentration ([Ca2+]i) regulates channel Po biphasically by changing the likelihood of the channel being in the high-Po mode and the mean closed duration (tc) of the low-Po mode. Even under optimal ligand conditions ([Ca2+]i = 2 µM and cytoplasmic InsP3 concentration [InsP3] = 10 µM), the maximum mean Po observed was only ∼ 0.1, with the channel being in the low-Po mode the majority of time. InsP3 (33 nM to 10 µM) activates the channel by reducing its sensitivity to Ca2+ inhibition. In experiments with cytoplasmic-side-out nuclear membrane patches, when initial [Ca2+]i was low (55 nM) and [InsP3] was rapidly dropped from 10 µM to 0 using perfusion solution exchanges, the InsP3R channels had a high propensity to transiently enter a more active state before they became inactivated by the removal of InsP3. This behavior was more prominent, with the channel Po transiently increasing to ∼ 0.6, when [Ca2+]i was increased to optimal level (2.1 µM) instead of remaining constant at 55 nM. No transient increase in Po was observed in similar [InsP3] drops with [Ca2+]i kept constant at 2.1 µM. This suggests that sub-saturating [InsP3] can induce type 1 InsP3R channels to enter the high-Po mode with higher probability than saturating [InsP3].