This paper presents a new computer program called Zigzag, suited for investigating readers' mental activity during text processing. The program permits data collection of two different kinds, namely reading time and reading errors for each of the words read. The second dependent measure, reading errors, provides a better control of the readers' text comprehension and permits the restriction of statistical analyses of the time data to correctly identified words. Subjects of this study were French-speaking third and sixth graders living in the greater Montreal (Quebec, Canada) area. One major goal was to examine the effect of visually presented contextual information on text processing at two developmental levels and to examine the readers' dependence on visual information in an interactional perspective. The paper analyzes the results for the experimental text as a whole, but also with respect to some linguistic variables, such as grammatical category, word position in the sentence, or other variables. Results indicate major differences between the data and the time data obtained by means of other on-line techniques with respect to the same linguistic variables. The paper concludes with a more detailed discussion of these differences. Four tables and two figures of data are included. Contains 27 references. (RS)