Seasonal fluctuations of dry matter, water soluble carbohydrates and total nitrogen content were traced in underground storage organs of Cirsium arvense and Tussilago farfara. Samples from experimentally planted monocultures of the two species were analysed. Both species showed identical fluctuations in dry matter and carbohydrate contents. A gradual decrease in autumn and winter, succeeded by a rapid decline in spring, after a distinct minimum in April-May was followed by a steep rise towards a maximum in late summer. Fluctuations seem bo be affected by weather conditions during the growing season. The nitrogen content decreased gradually from early spring until the middle of the summer and then increased similarly until the start of the cycle.