The developmental time-course of type I and type II benzodiazepine receptors in the chick optic lobe was determined using a triazolopyridazine, CL 218872. At embryonic day 13 most of the binding sites corresponded to type II (98.23%), while type I represented only a minor proportion (1.77%). During development there was an increase in type I binding sites which reached 62.88% in adulthood, while type II binding sites decreased to 37.12%. These results demonstrate a differential ontogeny of two benzodiazepine receptor subtypes. Changes in the benzodiazepine binding population may account for the variability in the GABA-benzodiazepine receptor interaction during chick optic lobe development.