It was aimed to assess if intrathecal (i.t.) injections of acetylsalicylic acid and salicylic acid depress C fibre-evoked activity in the sensory part of the nociceptive system. In rats under urethane anaesthesia, activity was elicited in single neurones in the dorsomedial part of the ventral nucleus (VDM) of the thalamus and in ascending axons of the spinal cord by supramaximal electrical stimulation of the sural nerve. Acetylsalicylic acid and salicylic acid injected i.t. significantly reduced the activity evoked in thalamic neurones. The maximum depression amounted to about 50% of the activity evoked in the controls and was produced by acetylsalicylic acid at a dose of 50 micrograms (0.28 mumol)/rat and by salicylic acid at a dose of 37.5 micrograms (0.27 mumol)/rat. Indomethacin injected i.t. also reduced C fibre-evoked activity in the thalamus in a dose-dependent fashion, 100 micrograms producing a 50% depression. Salicylic acid (37.5 micrograms/rat, i.e.) depressed C fibre-evoked activity in ascending axons but had no effect on A beta fibre-evoked activity. It is concluded that i.t. injection of acetylsalicylic acid selectively inhibits nociceptive impulse transmission in the spinal cord by an action of the salicylic acid moiety. It is possible that prostaglandins are involved in the central action of salicylic acid.