Epidemiological evidence suggests that hypertension and coronary heart disease are programmed by exposure to a poor diet during intrauterine life. It has been proposed that the prenatal environment may exert an adverse effect on the development of the kidney and hence later control of blood pressure. These assertions are supported by animal experiments. In the rat, fetal exposure to a maternal low protein diet is associated with disproportionate patterns of fetal growth and later elevation of blood pressure. Pregnant female rats were fed control (18% casein) or low protein diets throughout pregnancy, or during specific periods. Nephron number was determined at day 20 gestation, full term and 4 weeks of age. Exposure to low protein throughout gestation, or in mid-late gestation increased total nephron number at day 20. By term nephron number was reduced, relative to controls, in rats that were undernourished between days 8-14 or 15-22 gestation. At 4 weeks postnatally rats exposed to low protein throughout fetal life had a reduced (13%) nephron complement and blood pressures 13 mmHg above control animals. Lower renal size and elevated blood pressure persisted to 19 weeks of age, at which time glomerular filtration rate was normal. The data are consistent with the hypothesis that maternal undernutrition may programme the renal nephron number and hence impact upon adult blood pressure and the development of renal disease.