Adenosine and analogues, and antagonists to adenosine and putative excitatory amino acid transmitters were topically applied to the cerebral cortex of urethane-anaesthetised rats and their effects on the somatosensory evoked potentials (SEPs) examined. 2-Chloro-adenosine decreased the amplitude of the SEPs whereas adenosine did not. Both L-(-)N6-phenyl-isopropyladenosine (L-PIA) and 5'-N-ethylcarboxamide adenosine (NECA) depressed the SEPs; the effect of L-PIA was more marked than that of NECA. 8-p-Sulphophenyl theophylline increased the amplitude of the SEPs and also inhibited the effects of 2-chloro-adenosine and L-PIA. Kynurenic acid decreased the amplitude of the SEPs. The results suggest that the initial component of the SEP is a post-synaptic event and that endogenous adenosine probably modulates thalamo-cortical synaptic transmission.