A study of barley sprayed at six different stages of development with radionuclides of Cr, Fe, Co, Zn, Hg and Pb. The initial retention followed the equation: IR= 1—e−0·31 g.cmstaggered−1, where g is the dry weight of the herbage in a plot in grammes, and cm the height of the plants at spraying. The field loss depended on the growing rate and the development of the plants and thus varied throughout the growing period. The field loss coefficient λ was 0·054 days−1 in the first part of the period and 0·017 days−1 in the second part. If the whole growing season is considered, the loss of activity in per cent is about equal to the time in days from the spraying to harvest. Zn and Co showed the highest concentrations in the grain. The translocation of Cr, Pb and Hg within the plant was small. Fe was translocated to the grain as was Zn, but to a far less extent.