A study of the use of the complex Finnish morphological rule system in 45 children, aged 20-24 months, examined the children's inflection of nouns and verbs in speech characteristic of everyday Finnish. Analysis of the correct, unanswered, and incorrect test items found six classes of errors, which were then examined for clues to the underlying psychological processes of the children's speech. The subjects were divided into three speech achievement levels for further error pattern and rule application analysis. Results indicated that the children did not apply the same rule to all morphological forms, with rule selection seemingly guided by the morphological mastery level, form-specific variables, and contextual cues of the test items. It is concluded that further research in the interaction of these variables in different contexts is needed. (MSE)