Methylamine influx into Hydrodictyon has been measured with [l4C]methylamine. The influx increases with rising external pH up to about pH 8. Between pH 8 and 9 influx remains quite constant, with a further increase above pH 9. Influx is light-dependent, temperature-sensitive, and is decreased by . During short-term influx (less than 4 h) metabolism of methylamine appears negligible. Prolonged influx results in CH3 accumulation, and efflux of K+, Na+, and H+. There is no effect on Cl− influx. Methylamine decreases the membrane electric p.d. by 60–120 mV at external concentrations of 0·2–1·0 mM.