We have studied the collateral sprouting of intact low-threshold mechanosensory nerves into adjacent denervated skin in rats. An "isolated" field was produced by extensive denervations of the surrounding skin; sprouting of the remaining cutaneous nerve supplying the field was looked for in the form of field expansion into the surrounding denervated territory at various postoperative intervals. Such isolated fields failed to expand in the adult rat for periods up to at least 85 days. "Nonfunctional" sprouting is unlikely to explain this failure. However, similar experiments done in very young animals gave a different result. In rat pups aged less than 20 days, isolated fields did expand, but this ceased at about 20 days, and attempts to evoke it after this time were unsuccessful. There seems to be a critical period for sprouting of these touch-sensitive nerves into denervated skin, and our evidence suggests that it may not begin until about 15 days of age. Within this developmental window the sprouting that occurs is spatially constrained, an isolated field expanding preferentially into denervated skin of the parent dermatome; if only skin of neighboring dermatomes is available there is no expansion. In contrast, low-threshold nerves regenerated readily after a crush at all ages studied, and the mechanosensory fields established by regenerating nerves expanded progressively into denervated skin without apparent constraints at dermatomal boundaries. The temporal and spatial constraints found for the sprouting of intact low-threshold axons are in marked contrast to their absence for the well-described sprouting of high-threshold (nociceptive) nerves.