Measurements of the integrated atmospheric radio noise field strength on 27 kHz at the Observatory of Kühlungsborn (11.8°E, 54.1°N)in the period 1952–1990 have been analysed to give a contribution to the long-term variation of thunderstorm activity over the Amazon basin during southern summer. In accordance with the known climatic trends, the long-term variation consists of a downward (1952–1974) and an upward (1974–1990) period. Episodes of El Nin̄o Southern Oscillation and the increasing rate of deforestation during recent decades have an impact on the variability of the thunderstorm activity. For the year-to-year variation of the noise level a mean magnitude of 7.5% and a maximum of 20% were determined, which describes the change of production of nitrogen oxides caused by varying thunderstorm activity.