Awareness of H2S as an important, widespread signalling molecule is growing rapidly. It is known to influence, for example, proliferation of vascular smooth muscle cells and pain sensation (1). Interestingly, both of these processes involve T-type Ca2+ channel activity (2;3). We have therefore investigated whether H2S modulates T-type Ca2+ channels, using whole-cell patch clamp recordings from HEK293 cells over-expressing, separately, T-type Ca2+ channels Cav3.1, 3.2 and 3.3 (4). Bath application of NaHS, which produces H2S in solution, caused a concentration-dependent inhibition of currents in Cav3.2-expressing cells. Inhibition was observed at all activating test potentials, and all subsequent values are of currents recorded at a test potential of -20mV (holding potential -80mV). Maximal inhibition of 35.3±2.2%, n=6, P<0.0001, student’s paired t-test) was observed at the NaHS concentration of 1mM. By contrast, current carried by Cav3.1 channels were unaffected over the same concentration range, and those carried by Cav3.3 were only modestly inhibited (13.2±3.6%, n=5, P=0.022) by 1mM NaHS. Thus, H2S appeared to inhibit Cav3.2 channels selectively. Inhibition of Cav3.2 was only poorly reversible, but could be reversed by dithiothreitol (2mM; n=7), suggesting H2S acts via channel redox modulation. Given the high selectivity of H2S for Cav3.2, we examined its effects in H191Q mutant Cav3.2-expressing cells, since this extracellular histidine residue is known to be essential for redox-sensitive binding of divalent metals (3). These mutant channels were insensitive to H2S (1mM NaHS, n=6). In agreement with previous studies (e.g. (5)), the zinc chelator TPEN (10µM) augmented Cav3.2 currents, by 150±9.0% (n=6), and in its presence NaHS was without significant effect on currents carried by Cav3.2 channels. Our data indicate that H2S selectively inhibits Cav3.2 T-type Ca2+ channels heterologously expressed in HEK293 cells. Inhibition requires the extracellular histidine residue H191. Our results do not, however, support the previous suggestion that H2S might augment T-type Ca2+ channel activity by acting as a zinc chelator (reviewed in (3)).