Motor axons can form sprouts from their terminal arborizations in response to partial denervation, and when exposed to pharmacological blocking agents like TTX, botulinum toxins alpha-bungarotoxin, or curare. Each of these experimental procedures has cessation of muscle contractile activity as a common feature. We tested the specific role of muscle fiber inactivity in regulating nerve terminal sprouting by chronically treating adult frog (Rana pipiens) cutaneous pectoris muscles with formamide. Exposure to formamide, unlike the other compounds used to study sprouting, selectively inhibits muscle contractions without blocking pre- or postsynaptic transmission or muscle fiber action potentials. Repeated formamide applications were used to achieve chronic block of muscle contractile activity in vivo for up to 6 weeks. Motor axons in formamide-treated inactive muscle sprouted only from their terminal arborizations, but not from nodes of Ranvier. The onset of this sprouting was protracted compared with that seen in pharmacologically blocked mammalian muscles, and sprouts in formamide-treated muscles were more complex and ornate. The frequency of sprouting terminals was less in these formamide-treated muscles than that seen after alternate methods of contractile block, and this suggests that contractile inactivity alone serves as only a moderate cue for sprouting. The possibility is discussed that the prolific sprouting seen following neurotoxin administration may, in fact, be due to perturbations in synaptic transmission or muscle electrical activity rather than muscle fiber inactivity.