[en] Male mice, heterozygous for the Rb(11.13)4Bnr translocation, were irradiated for 14.5 min with either a dose of 15-rad fission neutrons or 60-rad X-rays. Animals of this karyotype are known to show high levels of spontaneous autosomal non- disjunction (20-30%) after anaphase I. The effects of the irradiation on this process were determined after 2 and 3 h in air-dried preparations. The length of the period from the end of meiosis I till the end of meiosis II was assessed autoradiographically, with the aid of cells showing a labelled Y chromosome only and appeared to last less than 3 h. Inter-mouse variation with regard to the duration of the period last premeiotic S-phase till diakinesis/metaphase I prevented a more accurate estimate. On the basis of this 3-h datum, the induced effects were studied at intervals of 2 and 3 h after the start of the irradiation. The influence of irradiation was assessed by scoring: (1) univalents in primary spermatocytes, (2) delections, aneuploid chromosome counts and precocious centromere separation in secondary spermatocytes, and (3) chromatid gaps and breaks in both cell types. Both radiation types induced comparable levels of chromosomal damage. A neutron X-rays RBE value for these parameters was calculated to be 5.4 for the MI stage and 3.3. for the MII stage. The significantly higher incidence of cells showing damage at MII than at diakinesis/MI is not believed to indicate a difference in radiation sensitivity, but is believed to be merely the consequence of the different chromosomal processes taking place during the irradiation taking place during the irradiation-fixation time interval. (orig.)