Several enzyme activities localized in the luminal compartment of the endoplasmic reticulum require the transmembrane fluxes of their substrates, products and/or cofactors. Accordingly variety of structurally unrelated compounds is (or should be) able to cross the ER membrane, but only a few transporters have been reported so far. It has been repeatedly observed that microsomal vesicles derived from the endoplasmic reticulum exhibit a basal permeability towards various compounds – including xenogenous molecules – which presumably do not have strictly specific transporters. Nevertheless, it might have a physiological function as a low affinity, low capacity transporter with low selectivity. The pore of the translocon complex in the endoplasmic reticulum is large enough to be permeated by small molecules and there are more and more data about the permeation of small compounds through the translocon. Previous works on cell experimental systems indicated that a small, neutral molecule and Ca2+ permeate the peptide channel. Our experiments on rat liver microsomes confirmed the role of translocons as Ca2+ leak channels. In addition we proved the role of translocons in the non-specific transport of a small, neutral molecule (sucrose) and anions (UDP-glucuronic acid, hexose phosphates). The translocon mediated transport could be increased by puromycin, which inhibits elongation and causes premature releasing of the nascent peptide from the ribosome. The effect of puromycin could be prevented by addition of puromycin antagonist and an antibody against the translocon complex. The non-specific transport – due to the occurrence of translocons – was more explicit in the rough microsomal subfraction. Our results show that the translocon peptide channel contributes to the non-specific Ca2+ leak. The translocon mediated transport can ensure the substrate supply of intraluminal ER enzymes, and can provide counterligands for known ER specific antiporters.