In the nematode Caenorhabditis elegans six GABAergic motor neurons, known as DDs(1,2), remodel their patterns of synaptic connectivity during larval development(3). DD remodelling involves a complete reversal of the direction of information now within nerve processes without marked changes in process morphology. We used a marker localized in vivo to DD presynaptic zones to analyse how the timing of DD remodelling is controlled. In wild-type animals, DDs remodel their synaptic outputs within a 3-5-hour period at the end of the first larval stage. We show that the heterochronic gene lin-14, which controls the timing of stage-specific cell lineages(4,5), regulates the timing of DD synaptic output remodelling. In lin-14 loss-of-function mutants, DDs remodel precociously. The degree of precocious remodelling is correlated with the level of lin-14 activity. Expression of lin-14(+) in the DDs of lin-14-null mutants rescues the precocious remodelling, indicating that lin-14 can act cell-autonomously. Consistent with this hypothesis, LIN-14 protein levels decrease in the DDs before remodelling. Our observations reveal a role of heterochronic genes in non-dividing cells, and provide an example of cell-autonomous respecification of neuronal connectivity.