Recently, two compounds have been developed, designated septide and senktide, which are highly selective agonists for the substance P receptor, types NK-1 and NK-3, respectively. Each of these, when injected intrathecally in awake rats, produced a distinct and non-overlapping constellation of sensory and behavioural effects which were subsets of the symptoms evoked by intrathecal administration of substance P. Prior systemic administration of 5-hydroxytryptamine (5-HT), alpha-adrenergic and opiate receptor antagonists, at doses sufficient to block the behavioural effects of the corresponding receptor agonists, did not alter responses to intrathecally injected septide or senktide. This was so, even for symptoms which suggested inhibitory mediation, hypoalgesia and (transient) motor flaccidity. Septide and senktide, administered by lumbar puncture and by indwelling catheter, produced identical results. Finally, in contrast to some other peptides, flaccid paralysis induced by senktide was not accompanied by spinal necrosis.