We studied the distribution of calretinin immunoreactivity (CR-ir) in the rostral and intermediate levels of the spinal cord of lampreys from embryonic to adult periods. CR-ir was first observed at hatching in motoneurons and primary sensory neurons of the spinal cord, the dorsal cells. During the prolarval period two new cell types showed CR-ir: ganglion cells and interneurons. Motoneurons, dorsal cells, and ganglion cells were strongly positive, whereas interneurons were weakly stained in late prolarvae. The intensity of CR-ir in these four types of cells changed during the larval period. Increase of CR-expression was found in interneurons but a decrease in dorsal cells and in ganglion cells. These changes were more evident in premetamorphic larvae. Postmetamorphic lampreys showed almost no CR-ir in dorsal cells. In adult lampreys, the interneurons showed the highest CR-ir, whereas motoneurons were more weakly stained than in earlier stages of development. Moreover, in adults the dorsal cells and the ganglion cells showed no CR-ir. The present study shows that CR-ir changes during lamprey spinal cord development in different types of neurons, sometimes in opposite ways. This plasticity of CR-expression may indicate different needs from subsets of lamprey spinal cord cells involved in the different locomotor behaviors along its life cycle.