Slo3 , a pH-sensitive K+ channel, is closely related to the large-conductance Ca2+-activated K+ channel, Slo1(1). Slo3 has been the subject of very few studies and little is known about its function aside from a vital role in the capacitation-induced hyperpolarization of spermatozoa (2). Slo3 sequences have been detected in a range of tissue cDNA libraries as expressed sequence tags (EST). We investigated the sequences of Slo3 cDNA in mouse brain to identify potentially novel neuronal splice variants of the Slo3 channel. Analysis revealed most of these to have out of frame coding regions; however two of the EST’s from brain (CV562866 & AK083174) have an open reading frame encoding the final 381 amino acids of the Slo3 C-terminus (CT). The CT product contains the epitope for the anti-Slo3 antibody (Neuromab), and was detected by Western blotting when expressed in HEK 293 cells. Proteins of a similar size (≈47kDa) were also detected in Western blots of membrane and supernatant fractions from protein isolated from a range of rat tissues. Functional effects of the CT protein were studied by expressing Slo3 and Slo1 subunits in Xenopus oocytes and recording membrane currents using two-electrode voltage clamp. Co-expressing CT significantly reduced Slo3 current evoked by a depolarising pulse to +140 mV from 4.99 ± 0.26 µA (n=20) (mean ± standard error) to 4.26 ± 0.33 µA (n=18) (p<0.05, Students t-test). Increases in Slo3 currents by Sloβ4 co-assembly (3) were eliminated by CT expression. Co-expressing Slo3 CT fragment with Slo1, however, increased currents evoked by a depolarising pulse to +140 mV from 5.51 ± 0.28 µA (n=37) to 6.54 ± 0.36 µA (n=37) (p<0.05, Students t-test). These findings suggest that Slo3 is expressed in a variety of tissues, including brain, as a C-terminal fragment. This splice variant lacks the transmembrane domains as well as the RCK1 domain of the full length Slo3 K+ channel. Interestingly the splice variant appears to regulate expression of both Slo3 and Slo1 at the cell membrane. Furthermore discovery of this variant may explain some of the non specific binding of the anti-Slo3 antibody when probing for the full length Slo3 channel.