The present report deals with experiments to ascertain whether cells, when in states of physiological activity, are more susceptible to irradiation than when they are not. The tissue used was from the mesonephros of nine- or ten-day chick embryos grown in tissue culture. The amount of irradiation necessary to produce an effect was found to be extraordinarily high. There is a latent period and the intensity of the reaction depends upon the dosage and the condition and type of the cells. Investigations of the past few years in this laboratory (1) have shown that fragments of kidney tubules of the chick mesonephros continue to function in tissue culture. The secretory activity of the proximal tubules is strikingly shown by their ability to accumulate phenol red from the medium This suggested a means of testing the effect of x-rayon the secretory mechanism. The cultures were prepared by placing teased fragments of the mesonephros in hanging drops of a mixture of fowl plasma and Tyrode solution on coverslips and incubating at 37.5° C. After a few hours the cut ends of the tubules heal over, converting the tubules into closed segments which, when secretion occurs, become progressively distended. In some cases the epithelial cells at the cut ends grow out to form sheets extending beyond the explant. In cultures of twenty-four to forty-eight hours the closed segments of proximal tubules within the explanted fragment become distended with secreted fluid. With plasma alone the distention is distinctly less than when phenol red or magnesium sulphate is added to the medium. In the latter case the distention is taken to indicate an enhancement of the secretory process. The outgrowth from the explant consists of wandering cells, fibroblasts, and sheets of proliferating epithelium. The cultures thus provide for a study of epithelial tissue as organized tissue in the form of secreting tubules and as unorganized tissue in the form of outgrowing sheets. The exposure of the cultures to x-ray treatment necessitated subjecting them to room temperature for about one to two hours. This causes a temporary cessation of secretory activity (1), which is speedily resumed on returning the cultures to body temperature. The controls and experimental cultures were always submitted to the same change of temperature. The x-ray equipment was a dual tube, self-rectifying machine. This provides two cross-firing beams of x-rays of high intensity and large doses can be administered in a few minutes’ exposure. The secondary voltage was kept at 200 kv. and the current passing through each tube was 30 rna. The output intensity, measured in air at the point of treatment (T. S. D. 14.8 cm.) after filtration by the heavy glass tube wall, was approximately 2,700 r/min. In general, these tissue culture studies deal with the effect of irradiation on secreting and non-secreting renal epithelium, and on other activities of cells, including motility and regenerative processes.