Chlorophylls were quantitatively studied in the leaves of black locust (Robinia pseudoacacia L.) seedlings exposed to electromagnetic fields of high frequency. Exposure system was designed and built up to make possible simultaneous exposure of seedling lots (3 months old) to low power density electromagnetic fields corresponding to a frequency of 400 MHz. After three weeks of daily exposures (1, 2, 3 and 8 hours), chlorophyll levels were measured using adequate spectral device. Statistical analysis of experimental results was performed by means of t-test to identify significant modifications induced by electromagnetic treatment in exposed samples in comparison to the control. Chlorophyll-a as well as chlorophyll-b level was found to decrease except the exposure time of two hours, where a considerable enhancement was noticed. It was revealed that the ratio of the two main types of chlorophyll was decreasing logarithmically to the increase of daily exposure time.