Cadmium effects were tested on 50-d-old laboratory-grown sporophytes of the kelp Laminaria saccharina under continuous-flow laboratory conditions. In standard 6-d experiments the rapid rate of growth of the plants (30 '/o length increase d.' in controls) provided a suitable parameter for the measurement of cadmium effects. Over a 6-d period a concentration of 4.5 pprn Cd was lethal. There is evidence that a lower concentration would be lethal over a longer period of exposure. In the range of 0.2 to 4.5 pprn Cd the reduction in growth rate with increasing Cd concentration was nearly linear. The growth rate diminished to 50 O/o of the controls at approximately 2.15 pprn Cd. When plants exposed to cadmium for 6 d were measured after a further 8 d in unpolluted seawater, the concentration causing growth-rate reduction to 50 V0 of the controls was 0.86 pprn Cd. Long-term after effects are more serious than is immediately evident and exposure time is more important than concentration. Cadmium uptake begins almost immediately and is apparently unregulated, indicating potentially very high tissue concentrations. Over 100 ng Cd mg dry wt -' was accumulated by plants in 6 d from a 0.78 pprn Cd solution. Saturation was not reached. Relatively more cadmium is taken up from lower ambient concentrations. Slower-growing plants and slower-growing regions of the thallus (stipe/holdfast; distal blade region) take up relatively more cadmium. At concentrations over 2.3 pprn Cd the blades show a sharply delimited dlstal loss of pigment. Cadmium inhibits photosynthesis, as well as dark carbon assimilation. The degree of inhibition depends on time and concentration. In distal plant regions the photosynthetic potential is more sensitive to cadmium. The reduction in photosynthesis in the most actively growing region with increasing cadmium concentration follows a curve very similar to that for the reduction in growth. The decrease in growth and photosynthetic potential caused by cadmium is not correlated with pigment loss or cadmium accumulation.