Chicks that peck a small bright bead coated in a distasteful substance can learn in a single trial to subsequently avoid a similar bead. The taste aversant commonly used is methyl anthranilate, which also has a strong pervasive odour. We have compared the efficacy of methyl anthranilate and the apparently odourless quinine as aversants. Methyl anthranilate-trained chicks learnt the task and the memory apparently persisted undiminished for at least 24 h. Quinine-trained chicks exhibited a memory for the task similar to that of methyl anthranilate-trained chicks 45 min after training, this thereafter declined until, at 24 h after training, they showed no recall. We investigated the incorporation of a radio-labelled synaptic membrane glycoprotein precursor, [3H]fucose, into three regions of the chick forebrain; two of these regions have previously been implicated in learning using methyl anthranilate as the aversant. There was a significant increase in [3H]fucose incorporation into the left lateral cerebral area and numerically similar, but nonsignificant, increases in the intermediate part of the medial hyperstriatum ventrale and lobus parolfactorius. There were no such increases in the right hemisphere of methyl anthranilate-trained chicks or any region of either hemisphere of quinine-trained chicks. Thus, the memory for methyl anthranilate is longer-lasting than that for quinine and is associated with increased fucosylation in the left cerebral hemisphere and although in the short-term, chicks can retain a memory of the one-trial passive avoidance task with quinine as the aversant, this does not result in a localized increase in cerebral [3H]fucose incorporation.