Normally, women become refractory to the pressor effects of infused angiotensin-II (A-II) early in pregnancy. Gravid women destined to develop pregnancy-induced hypertension (PIH) lose this refractoriness to A-II several weeks prior to the detection of hypertension. Normal pregnant women also lose their A-II refractoriness after treatment with prostaglandin synthetase inhibitors and, in this regard become similar to gravid women who are destined to develop PIH. From this observation, we have concluded that a prostaglandin(s) or a prostaglandin-related substance(s) is likely involved in the mediation of vascular reactivity to A-II during pregnancy. The present study was conducted to ascertain if control of vascular reactivity during pregnancy also involves the cyclic nucleotides. Since theophylline is known to inhibit the action of phosphodiesterase, an action that results in increased cellular levels of cyclic 3',5'-adenosine monophosphate (cAMP), we evaluated the effective pressor dose of A-II before and after the administration of theophylline to women with mild PIH who were beyond the 28th week of gestation. The effective pressor dose of A-II in these women with PIH before theophylline treatment was 7.3 +/- 1.4 ng. times kg.(-1) times min.(-1) (mean and standard error). Following treatment of these women with the equivalent of 500 mg. of theophylline daily for four days, the effective pressor dose of A-II was 16.7 +/- 3.8 ng. times kg.(-1) times min.(-1) (p less than 0.025). These findings are consistent with the view that a prostaglandin(s) synthesized in the arteriole may modulate the vascular refractoriness to A-II by altering the intracellular level of cAMP in vascular tissues.