omega-GVIA conotoxin has been used to mark presynaptic N-type voltage sensitive calcium channels (VSCC). Litzinger used omega-conotoxin binding to describe a critical period of neurodevelopment in Swiss Webster mice between postnatal days (PND) 11 and 14, which appears to be important to the initiation of proper final development of the central nervous system. In this study, the authors compare how three different omega-conotoxins (i.e. GVIA from Conus geographus, MVIIA from Conus magus, and RVIA from Conus radiatus) mark N-type VSCC during this critical period in Swiss Webster mouse cortex. 125I-GVIA was bound to Swiss Webster mouse cortex synaptosomal membrane fractions at postnatal days 8 and 14. 125I-GVIA binding displacement curves were obtained by incubating membranes with increasing concentrations of unlabeled GVIA, MVIIA, and RVIA. Displacement curves and IC50 were calculated for each of these three omega-conotoxins, and then compared. At PND 14, GVIA, MVIIA and RVIA were able to displace greater than 95% of 125I-GVIA binding. At PND 8, however, MVIIA was only able to displace 83% of 125I-GVIA binding, and RVIA was only able to block 84%. The IC50 does not appear to change significantly during this period of development for any of the omega-conotoxins. The inability of MVIIA and RVIA to completely block 125I-GVIA binding in pre-critical period Swiss Webster cortex denotes an alteration in the composition of N-type VSCC binding sites. With this data, the authors have suggested the presence of subtypes of the N-type VSCC in the cortex of pre-critical period Swiss Webster mouse.