The susceptibility of rats to alloxan undergoes a circadin rhythm. The toxicity rhythm, presumably involving injury to liver, kidney and other sites, pancreatic beta-cells in particular, is demonstrated in pooled data from 370 mature inbred Fischer or Minnesota Sprague-Dawley rats of both sexes kept in light from 06(00) to 18(00) alternating with darkness, some with free access to Purina laboratory chow with tap water at all times and some other rats subjected to one of three starvation schedules: 1) a 28-h fast before an intravenous alloxan injection; 2) a 28-h fast, except for a 4-h ad libitum feeding before injection; 3) a 28-h fast, except for a 4-h pre-injection tube-feeding of Nutrament (Mead and Johnson, Evansville, Indiana), 1.5 ml/100 g body weight. Survival time data on an additional 200 inbred Fischer rats reveal, next, that susceptibility to alloxan increases as the starvation span is lengthened from 24 to 84 h. The shortening in survival time indicative of this susceptibility increase is nonlinear; a circadian rhythmic change in susceptibility to alloxan is seen as a statistically significant wave-form indicative of the basic (persisting) rhythm, of applied interest as well to students of experimental diabetes.