This and a previous investigation of the kinetics of tablet absorption [Folley, 1944] arose out of the use of the implantation technique of Deanesly & Parkes [1937] for the artificial induction of lactation in cows and heifers by prolonged oestrogen treatment [Folley & Malpress, 1944; Hammond & Day, 1944]. The requisite uptake of synthetic oestrogen (diethylstilboestrol or meso-hexoestrol) could be achieved by implanting a few large (1000 mg.) tablets or a considerable number of small (50, 25 or 15 mg.) tablets into the same site or 'pocket', but unabsorbed residues remained to be removed at the end of the treatment period. It was pointed out by Folley, Stewart & Young [1944] that the whole procedure would be simplified if conditions could be found under which tablets yielding a suitable daily dose of oestrogen would be completely absorbed in the required time, thus obviating the necessity for a second minor