Pregnancy is associated with increases in maternal cardiac output and plasma volume and a reduction in peripheral vascular resistance. Cardiac output and plasma volume are substantially reduced in pregnant rats fed a low-protein diet, but it is not known whether vascular function is also compromised. We have investigated vascular function in virgin and pregnant Wistar rats subjected to dietary protein restriction [9% (w/v) casein, compared with 18% (w/v) casein for controls]. The diets were fed to the groups for 18 days; in the pregnant rats, the diets were given from day 1 of pregnancy. Branches of the mesenteric arteries were studied on day 18 of the dietary period using myography. Significant reductions in sensitivity to acetylcholine occurred in vessels from virgin ( P = 0.04) and pregnant ( P = 0.01) rats that had consumed the 9% casein diet. In arteries from the virgin rats on the restricted diet there was also a significant reduction in sensitivity ( P = 0.0003) and maximum relaxation ( P = 0.009) to the NO donor spermine NONOate. Mean placental and fetal weights were significantly lower in the rats fed on 9% casein ( P <0.0001 and P = 0.005 respectively). Thus low-protein diets impair vasodilator responses in female rats. These effects may contribute to the poor cardiovascular adaptation to pregnancy and lower fetal weights associated with restricted protein intake.