Previous experiments of our group on the responvsiveness of rat group IV muscle afferents had shown that nerve growth factor (NGF) is an effective stimulant exclusively for nociceptive units. In the present study, we investigated the effects of the NGF-induced nociceptive input on dorsal horn neurones with intracellular recordings in vivo. In anaesthetised rats, dorsal horn neurones were tested with electrical and mechanical stimuli before and after injections of NGF (25 ml, 0.8 mM) into the gastrocnemius-soleus (GS) muscle. The results showed that of those neurones that reacted acutely to NGF i.m., the majority exhibited subthreshold excitatory postsynaptic potentials (EPSPs) but no action potentials (APs). After 5 to 30 min, some of these neurones showed changes in responsiveness in that they now fired APs instead of EPSPs during electrical GS muscle nerve stimulation. One day after a single NGF injection, the proportion of dorsal horn neurones responding with APs to mechanical stimulation of the GS muscle increased significantly. Most of these neurones had a low mechanical threshold. The data show that NGF-induced nociceptive input from muscle elicits mainly subthreshold potentials in dorsal horn neurones which are followed by a longer lasting sensitisation to electrical and mechanical stimuli.