Because comparable results were obtained using widely different EMFs, the blood-composition studies suggested to us that the EMF-induced alterations were mostly transient compensatory reactions of the body to a change in the electromagnetic environment. To determine the relation between magnitude and direction of the response and the conditions of application of the external EMF, we looked for changes in hematological parameters of mice due to short-term exposure to a full-body vertical 60 Hz electric field of 5 kv/m (13). To ensure maximum statistical sensitivity every mouse was sampled twice, once after exposure to the field for 2 days and once following a 2-day nonexposure period. There were four consecutive experiments, two with males and two with females. In each there were two groups: one for which the control period preceded the exposure period (nF-ðF), and one in which the pattern was reversed (F-ðnF). On “day 1” of each experiment the mice were divided into the two groups and the electric field was applied to one-half the population. On “day 3” the blood parameters were measured in each mouse and immediately thereafter the exposed and nonexposed groups were interchanged. On “day 5” the blood parameters were measured again and the mice were killed.