Brattleboro rats do not produce vasopressin (VP) because of a germ-line single-base deletion (di) that causes a frame shift downstream from the VP sequences and a loss of a stop codon. The resulting frame-shifted peptide precursor does not enter the secretory pathway in hypothalamic neurons, thereby blocking the neurosecretion of VP and other peptides. Yet, from birth onwards, a subpopulation of neurons in di/di rats slowly accumulates revertant cells with a hemizygous wild-type phenotype. Because the rate of reversion during aging is slowed by vasopressin infusion, it is of interest to consider these phenomena in relation to recent observations on 'adaptive' mutations in single cell bacteria and yeast that enable reversion of mutations that blocked cell division under conditions of nutrient deficits. In considering mechanisms that could produce revertant phenotypes in non-dividing cells of both pro- and eukaryotes, we note the pertinence of transcription-coupled repair and SOS 'error-prone' repair.