THE developmental time-course of type I and type II benzodiazepine rec
eptors in the chick optic lobe was determined using a triazolopyridazi
ne, CL 218872. At embryonic day 13 most of the binding sites correspon
ded to type II (98.23%), while type I represented only a minor proport
ion (1.77%). During development there was an increase in type I bindin
g sites which reached 62.88% in adulthood, while type II binding sites
decreased to 37.12%. These results demonstrate a differential ontogen
y of two benzodiazepine receptor subtypes. Changes in the benzodiazepi
ne binding population may account for the variability in the GABA-benz
odiazepine receptor interaction during chick optic lobe development.