According to Ellis (2008), motivation has received a lot of interest in language learning. It is unfortunate, however, that little attention has been paid to the dynamic nature of the construct, particularly changes in motivation within single classes and series of lessons. The study whose results are reported in this paper aims at investigating the dynamic character of foreign language motivation both within single lessons and sequences of classes over the period of several months. The participants were 52 senior high school learners. The study encompassed 121 naturally occurring English lessons. The data were gathered by means of a background questionnaire, motivational grids, observations and teacher's notes as well as individual interviews and lesson plans. The data were analyzed both quantitatively and qualitatively. The results of the study demonstrate that language learning motivation changes not only during a single class but also from one lesson to the next. Factors responsible for such variations included lessons, learners and school.