Dark-grown pea seedlings exposed to cyclic heat shocks or daily temperature changes undergo a morphogenetic development similar to that induced by far red light. The morphological changes observed include expansion of the leaves, shortening of the stems and opening of the hooks. Compared with control etioplasts, plastids of heat-treated seedlings are as large as fully mature chloroplasts and contain well developed, unstacked membranes. These morphogenetic changes correlate with elevated levels of SSU and LHCP mRNAs which, under these conditions, fluctuate in a circadian manner. In contrast, the ELIP mRNA remains under strict light control and shows circadian fluctuations only if the plants are exposed to a short period of illumination. We propose that periodic temperature changes, like light treatment, might serve as a 'Zeitgeber' signal for circadian rhythm. The data indicate a correlation between the existence of circadian oscillations and morphogenetic development.