Membrane depolarization of skeletal muscle cells induces slow IP3-mediated calcium signals in which dihidropyridine receptors act as voltage sensors (1) and that regulate the activity of transcription factors such as CREB, jun and fos (2), slow calcium signals with different kinetics can also be evoked by tetanic pulses (3) and several hormones (4). These signals appear unrelated to contraction and with a distinct nuclear component. Using primary cultures from neonatal rats (humanely killed) skeletal muscle, we worked with both whole cells and with a preparation of purified, intact nuclei obtained after hypotonic shock and differential centrifugation. Western blot analysis, binding of 3-HIP3, immuno-gold labelled and fluorescent antibodies against all three types of IP3 receptors (IP3R) were used together with the calcium-sensitive dyes fluo-3, mag-fluo 4 or fluo 4-dextran were used both in confocal microscopy and fluorimetry to asses nuclear [Ca2+] changes. Both immuno-fluorescence and immuno-electron microscopy localized mainly type 1 IP3R in the nuclear envelope, type 3 IP3R in the nucleoplasmic region and type 2 IP3R in the cytoplasm when the number of particles in the different nuclear compartments was quantified, of more than 1500 particles counted, type 1 IP3R gold particles were found mainly in clusters of varying numbers (2 to 30); more than half of the gold particles were found in groups of the three or more particles, 72 % of the total count being in the vicinity of one or more particles. 47 % is associated to the nucleoplasm, 21% to the nucleolus, 25% to the inner nuclear membrane and 7% to the outer nuclear membrane. Type 3 IP3R gold particles were also near the inner nuclear membrane (20 %) whether 54%. Were found in nucleoplasm 26 % of particles were found in the nucleolus. Also in in clusters, reaching 78 % in relation to isolated particles. The outer nuclear membrane was not labeled. Isolated myonuclei responded to IP3 with transient [Ca2+] elevations. The same result was obtained with the 1B5 cell line, not expressing ryanodine receptors. Nuclear Ca2+ increase triggered by IP3 evoked CREB phosphorylation. These results may be correlated to those showing nuclear translocation of protein kinase C isoforms as well as PKC- dependent CREB phosphorylation (5). The ensamble of results support the idea that Ca2+ signals mediated by nuclear IP3R in myotubes are part of a distinct Ca2+ release component that originates in the nucleus and is likely to participate in gene regulation mediated by CREB.