This study analyzed the grammatical features of two classes of words in Japanese, adjectives and adjectival nouns. Both have functions similar to those of English adjectives, but their behaviors differ syntactically or morphologically from each other. Differences in psychological processes, evident in both lexical retrieval processes and native speakers' errors, are examined. Data are drawn from two sources: elicited "tip-of-the-tongue states" and naturally occurring speech errors in sentence production. Subjects for the first data elicitation were 24 native Japanese speakers, who were provided with 50 written definitions (25 adjectives and 25 adjectival nouns) and asked to give specific information about each. Data in the second analysis were derived from a corpus of 3,300 naturally-occurring speech errors Results support a dual-stage model of lexical retrieval assumed in many sentence production models, but also emphasize the distinctiveness of the two categories during the early stages of sentence production, suggesting that the differences between adjectives and adjectival nouns are syntactic. Contains 18 references. (MSE)