Calmodulin (CaM), the principal mediator of the calcium signal, regulates numerous processes pertinent to neural function. Mammalian CaM is generated from three genes that give rise to five distinct transcripts. To determine the regulation of individual CaM transcripts in neurons, we assessed their abundance during differentiation of human IMR-32 neuroblastoma cells. Northern analysis revealed that the 4.1 kb CALM1 transcript was specifically upregulated about two-fold during differentiation, and that this increase correlated with neurite extension. By contrast, the CALM2 and CALM3 mRNAs as well as the 1.7 kb CALM1 transcript showed an initial increase but then returned to levels close to, or only slightly above, controls. The increase in the 4.1 kb transcript was largely due to its specific stabilization in differentiated cells. However, total cellular CaM levels did not change significantly throughout differentiation. To begin to address whether the 4.1 kb CALM1 transcript might play a unique role in providing local CaM pools, we determined its localization in differentiated IMR-32 cells using in situ hybridization. The 4.1 kb CALM1 transcript localized to the cell body, but was also present within extending neurites. This finding agrees with in vivo studies showing elevated levels of the 4.1 kb CALM1 transcript in adult rat central neurons and the presence of CALM1 transcripts in dendrites, and establishes a human in vitro model system to study individual CaM transcripts with respect to neuronal functions.