TRIC-A and TRIC-B are two homologous proteins found in the sarcoplasmic/endoplasmic reticulum (ER/SR) of various tissues and they function as monovalent cation-selective ion-channels (1, 2). Genetic deletion studies have indicated a crucial role of these channels in intracellular Ca2+-handling in different cell types. Mice lacking both TRIC-A and TRIC-B die of heart failure at embryonic stages and display serious defects in SR Ca2+-release (1) while knockout of the TRIC-B isoform severely impairs intracellular Ca2+-homeostasis in alveolar cells and results in neonatal death (3). A TRIC-B deletion mutation has also been recently linked to the genetic disorder, osteogenesis imperfecta (4). Identifying the biophysical properties and the possible physiological regulators of the TRIC channels is essential for understanding their individual functions in different tissues but investigation is complex because both isoforms are present in most tissues. We have therefore isolated SR vesicles from Tric-a- knockout mouse skeletal muscle and incorporated them into artificial membranes under voltage-clamp conditions as previously described (5) in order to investigate regulatory gating mechanisms of native TRIC-B channels. We obtained TRIC-B single-channel recordings in K+ containing solutions in the presence of 10 µM free Ca2+ at various pH and at holding potentials in the range ±50 mV. Our single-channel data suggests that TRIC-B channels always open to the full open state via transitions to sub-conducting open states. We demonstrate that TRIC-B gating to the full open state and to sub-conductance open states is steeply voltage-dependent. At negative potentials, Po in the full open state was approximately 0 and few current fluctuations to sub-conductance levels were observed. Steep channel activation occurred at positive potentials between 0 and +20 mV while no further increase in Po was observed as the holding potential was raised above +20 mV. At physiologically relevant membrane voltages (0±10 mV), sub-conductance opening events accounted for approximately 75% of the total current flowing through TRIC-B channels, indicating that sub-conductance state gating may significantly contribute to physiological SR cation currents. In addition to voltage, we find that pH is another modulator of TRIC-B function. Cytosolic and luminal acidification markedly inhibited channel activity while alkaline pH on either channel side activated TRIC-B. For example, mean Po at +30 mV was decreased from 0.285±0.074 in symmetrical pH 7.2 to 0.099±0.028 (SEM, **p<0.01, n=8) after cytosolic pH was lowered to 6.2. This change in gating reflected a substantial increase in the proportion of openings to sub-conductance states.Our data suggests that TRIC-B channels would be constitutively active at physiological SR membrane potentials and that only a small voltage change across the SR would be sufficient to produce a maximal increase in Po. Further experiments are required to fully understand the complex mechanisms regulating TRIC-B channel opening and its precise role in the physiology and pathophysiology of SR Ca2+-release.