For demonstrating the changes of lectin biding pattern in the developing rat, nine biotinylated lectin were applied with ABC method. The affinities for SBA, PNA, RCA-1, sWGA, and Con A were observed on 15 day fetal rat, and an additional UEA-1 affinty on 17 day fetal rat was detected. From the next developing rats, lectin binding pattern was signicantly altered and these changes were greater in suckling rats than others. The notable changes of the duodenal mucosa in developing rats were as follows : A new affinities for SBA, sWGA and UEA-1 in the villous columnar cells and PNA, RCA-1, sWGA and Con A in the crypt columnar cells were detected from suckling rats. The SBA, PNA, BSL-1, RCA-1, sWGA and UEA-1 affinities in the Golgi zone of villous columnar cells showed a tendency to increase from the suckling rats, and a new affinities for SBA, PNA, BSL-1, RCA-1 and sWGA in that of crypt columnar cells revealed from same rats. All lectin affinities used except DBA and Con A were observed in the brush border and maintained a higher level until adults, however, no BSL-1 affinity was detected from the weaning rats. The UEA-1 affinity of goblet cells was detected from 19 day fetal rat and not observed from the weaning rats. And the affinity for DBA and SBA of the goblet cells was demonstrated from the suckling and weaning rats respectively. All lectin affinities examined were observed in the duodenal gland and increased from the suckling rats.