Adaptation in the avian olfactory bulb (OB) was examined by recording the activity of single OB neurones in the chicken (Gallus domesticus) during prolonged (120 s) odour exposure (20 ppm ammonia). The activity of each neurone before, during and after odour stimulation was investigated with analysis examining changes in firing rate and pattern. Of the 73 OB neurones recorded, 22 responded to ammonia stimulation with either inhibition or excitation of firing (both 50%). Their responses then either adapted (lessening of the response, 32%) or sensitized (heightening of the response, 68%) during the odour stimulus period. Both inhibited and excited units underwent adaptation or sensitization and their responses were consistent during odour stimulation. These results demonstrate the role of OB neurones in adaptive responses of the avian olfactory system during prolonged odour stimulation.