A study of the developmental expression of CR in the chick equivalent of the cochlear nucleus revealed that CR expression is not uniquely associated with the establishment of neuronal activity. Indded, removal of otocysts (primordium of the inner ear) of chicks did not influence the developmental expression of CR mRNA in the auditory brainstem. Conductive hearing loss produced by columella removal at early post hatching days (P1-7), also did not affect CR expression. These data suggest that intrinsic factors maybe of particular importance in the regulation of calretinin expression. Continuing studies examine the influence of posthatching cochlea lesions on calretinin mRNA expression and immunoreactivity. Initial results indicate less of an effect of cochlea ablation in young (P3) versus older (P7) chicks. In addition, immunoreactivity for CR may be increased in as little as 1 hour post lesion using an antibody to CR which preferentially recognizes the calcium bound form of the protein. These data suggest that the lesion produces a transient elevation of intracellular calcium which is not commiserate with changes in actual protein expression. Additionally, these data indicate that neuronal activity may play a greater role in regulating CR expression in mature than in the developing chick auditory system. Calretinin (CR) mRNA localization in the reticular formation (RF) of the rat and the colocalization of CR immunoreactive cells and glutamate receptors was examined. Results revealed populations of cells with unique combinations of CR and glutamate receptor immunoreactivity and showed that the occurrence of the glutamate receptors examined is not an adequate predictor of the colocalization of CR in RF cells.