Two related studies investigated (1) the extent to which native language input to five Japanese children was varied based on the children's age, and (2) the effectiveness of adult Japanese second language input to a three-year-old American child during a one-month period in Japan. In the first study, interactions of adult-child dyads were compared for children aged 2 years (n=3) and 4 years (n=2). Results suggest the younger children received more cues making native language linguistic patterns discernible, including shorter intonation units with ending rising pitch and more frequent use of bracketed utterances. The second study found the input a native English-speaking 3-year-old received was similar to that received by his native Japanese-speaking cousin: short intonation units with unique discourse devices such as a sentence-final particle "ne" and rising pitch. During 34 days, the child acquired several nouns, verbs, and other vocabulary, some words and phrases, a basic negative form, and various sentence-final particles often found difficult by non-native adults. After a month, the child was able to initiate a conversation with a native Japanese speaker, calling attention and changing the discourse topic by himself. Transcriptions and translations are appended. Contains 31 references. (MSE)