Increasing the atmospheric humidity in growth rooms increased growth of sugar beet in four experiments, of kale in two experiments and of wheat in three experiments. Growth of wheat was unaffected in another three experiments. The effects were variable and did not occur at all stages of growth. In general, decreasing the vapour pressure deficit by 5 mb (e.g. increasing the relative humidity from 70 to 90 per cent at 20 °C) increased dry weight by 20–30 per cent and sometimes by considerably more. Sugar beet and kale were more sensitive than wheat. Relatively, dry weight of tops was affected more than root weight and leaf area was affected more than total dry weight, so net assimilation rate decreased with increase in humidity. Water loss per plant depended on the vapour pressure deficit of the air, leaf area and species; it was only slightly affected by wind speed. Water loss per unit leaf area was less for wheat than for the other species and less for large than for small plants.