Tetanus toxin (TeTx) has been recently demonstrated to be a Zn2+-dependent endopeptidase that cleaves synaptobrevin, a protein in part responsible for neurotransmitter release. Nevertheless, certain aspects of TeTx action, for example, the causal relationship between TeTx and protein kinase C (PKC; EC 126.96.36.199) activity cannot be explained by this cleavage alone. In the present study, primary neurons from fetal rat brain, synaptosomes, and whole slices have been used to examine this issue. Low doses of TeTx (≤10−8M) caused PKC activity translocation in a manner similar to that produced by 12-O-tetradecanoylphorbol 13-acetate (TPA). TPA (≤10−7M) caused sustained PKC activity translocation, whereas TeTx produced translocation followed by relocation, depending on the dose and time of exposure. Immunoidentification with a monoclonal antibody recognizing both α and β isoforms revealed that TeTx induced moderate losses of PKC in the cytosolic fraction, without a comparable increase in the particulate fraction. Although moderate losses of activity were also noticed in the cytosolic fraction, the inconsistency with respect to activity translocation may be explained by translocation of additional PKC isoforms that are not identified by the antibody. Comparable levels of water-soluble inositol phosphate-labeled intermediates were obtained after treatment of cerebral cells and/or cortical brain slices with TeTx. Significant increases of 19 and 114% in the watersoluble myo-[2-3H]inositol-labeled inositol phosphate metabolites were found in cerebral cell culture and brain slices, respectively, after treatment with 10−8M TeTx. TeTx (10−8M) increased to the same degree the water-soluble inositol phosphate levels as did serotonin (10−5M) or carbachol (10−6M). It is suggested that part of the signaling cascade of TeTx consists of a component involving inositol phospholipid hydrolysis, which is associated with PKC activity translocation.