The presynaptic alpha-2 autoreceptors in pig brain cortex were subclassified in terms of alpha-2A, alpha-2B, alpha-2C and alpha-2D to test the hypothesis that alpha-2 autoreceptors belong predominantly to the alpha-2A/D pair of orthologous alpha-2 adrenoceptors. Slices of brain cortex were preincubated with [3H]norepinephrine and then superfused and stimulated electrically. pKd values of 13 alpha-2 adrenoceptor antagonists (including the partial agonist oxymetazoline) against the alpha-2 agoinst 5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline (UK 14,304) were determined. The stimulation periods used (six pulses at 100 Hz) did not lead to alpha-2 autoinhibition, as shown by the failure of all but one of the alpha-2 antagonists to increase the stimulation-evoked overflow of tritium. UK 14,304 caused a concentration-dependent decrease of the evoked overflow of tritium, with an EC50 value of 0.90 nM and a maximal inhibition of 95.2%. All antagonists shifted the concentration-inhibition curve of UK 14,304 to the right in a parallel manner. Antagonist pKd values were calculated from the shifts. The pKd values at the presynaptic alpha-2 autoreceptors in pig brain cortex correlated excellently with pKd values at previously subclassified alpha-2A sites but did not correlate significantly or correlated much less well with pKd values at alpha-2B, alpha-2C and alpha-2D sites. Also, ratios of Kd values of the antagonists at the presynaptic alpha-2 autoreceptors in pig brain cortex agreed well with ratios at previously subclassified alpha-2A sites but not with those at previously subclassified alpha-2B-D sites. It is concluded that the alpha-2 autoreceptors in pig brain cortex are alpha-2A, in accordance with the hypothesis mentioned.