Comprehension of reversible active and passive sentences was studied with 48 Japanese children between the ages of two and six. Four types of sentences were constructed using passive and active structures and two word orders: subject-object-verb (SOV) and object-subject-verb (OSV). The basic order of elements in a simple sentence in Japanese is SOV, but the order of the subject and the object can be interchanged to yield OSV. This reversal is possible because Japanese has postposed particles that mark grammatical role. To test comprehension with the four types of sentences, the children were required to act out the sentences using toy animals. In a production task, which immediately followed the comprehension task, subjects described what they saw on a slide viewer. Results suggest that children between the ages of three years, three months, and four years, two months, require a certain correlation between particles and their position in a sentence (particles are position-specific). Only children between five years, three months, and six years, two months, appear to begin freeing themselves from the constraints of word order and interpreting sentences based on the information conveyed by particles alone. A second experiment was conducted on the comprehension of complex active sentences, using the act-out procedure, with 39 children between the ages of three years, three months, and six years, two months. Findings suggest that the constraints of word order in comprehension of particles in Japanese children operate differentially, depending on the construction. (SW)