We have studied, by a nonisotopic in situ end-labeling (ISEL) technique, the frequency of apoptosis in the intestinal crypt cell of adult mice and in the external granular layer(EGL) of the cerebellum of fetuses by gamma-ray irradiation from 60Co or diagnostic ultrasound exposure. The extent of changes following 200 cGy(1090 cGy/min) was studied at 2, 4, 6, 8, 12, or 24 hours after exposure. The maximal frequency was found 4-8 hours after exposure. The mice that received 18, 36, 54, 108, 198, 396 cGy of gamma-rays or diagnostic ultrasound (7.5 MHz, 4.2 mW, ISPTA = 7.9 mW/cm2, IsppA = 114.3 W/cm2) for 10 or 30 minutes were examined 6 hours after irradiation. Measurements performed after gamma-ray irradiation showed a dose-related increase in apoptotic cells in each of the mice studied. The dose-response curves were analyzed with alpha linear-quadratic model: the frequency (number per crypt) of apoptotic cells in the intestinal crypt of adult mice was y = (0.0386 +/- 0.004204)D + (-0.0000535 +/- 0.00001120)D2 + 0.15475(r2 = 0.952, y = apoptotic cell per crypt cell, D = dose in cGy), and the frequency (percentage of apoptotic cell in the EGL) of apoptotic cell in the EGL of fetus was y = (0.1349 +/- 1.175)D + (-0.001522 +/- 0.334)D2 + 0.0477(r2 = 0.981, y = % of apoptotic cell in the EGL, D = dose in cGy). In the experiment of ultrasound exposure, the frequencies of apoptosis were 0.181 +/- 0.055(10 minutes exposure) and 0.325 +/- 0.294 (30 minutes exposure) in the crypt cells and 0.106 +/- 0.130% (10 minutes exposure) and 0.167 +/- 0.220%(30 minutes exposure) in the EGL. We estimated the relative dose of the yield from the experiment with ultrasound by substituting the yield from ultrasound exposure into the curue from the gamma-irradiation. The relative doses of ultrasound exposure compared with gamma-irradiation were 0.692 cGy(10 minutes exposure) and 1.334 cGy(30 minutes exposure) in the experiment for crypt cells and 0.432 cGy(10 minutes exposure) and 0.885 cGy(30 minutes exposure) in the experiment for EGL. Although there is presently no evidence to indicate that diagnostic ultrasound involves a significant risk, it is not wise to use diagnostic ultrasound indiscriminately.