- Thermodynamic properties of red cell lithium efflux were examined in pregnant women in relation to hypertension. Twenty-two normotensive women, 15 women with essential hypertension, and 27 with pregnancy-induced hypertension were studied. The rates of Li efflux at 37 degrees C in the three groups of pregnant women were similar and nondiscriminatory. The temperature dependence of the Li efflux, known to be uniquely modified in essential hypertension, allowed the differentiation of most (73%) of the pregnant women with essential hypertension as well. Among the women with pregnancy-induced hypertension, 63% showed a temperature-dependence pattern typical for normotensives, and they may be classified as patients with toxemia of pregnancy. The others (37%) showed a thermodynamic pattern of essential hypertension, but a follow-up study is required to ascertain whether they will indeed develop essential hypertension in the future.