The Lemna growth test is used to study the uptake of ¹³C-labelled organochlorines (pentachlorophenol, and trichloroacetic acid) into the plants as well as their effect on the incorporation of ¹⁵N labelled nutrients into the plants' nitrogen fraction simultaneously. A time-and a concentration dependent incorporation of ¹³C labelled Pentachlorophenol (10 to 1000 µg/litre) into Lemna gibba and L. minor and an inhibition of ¹⁵N incorporation into the plants was observed. The effect parameter ¹⁵N tracer incorporation was approx. twice as sensitive as the growth rate over 7 days. No incorporation of ¹³C labelled trichloroacetic acid (50 to 800 µg/litre) into L. gibba was to be observed within 24 hours and there was no effect on ¹⁵N incorporation either.