Undernutrition is implicated in linking in utero and postnatal growth to the development of cardiovascular disease in later life (Eriksson et al., 1999; Roseboom et al., 2000). We therefore investigated the effects of early gestation and postnatal undernutrition on cardiac morphology and left ventricular function in adulthood. Welsh Mountain ewes received 100% (group C, n=25) or 50% of global nutrient requirements (group U, n=26) from conception to day 30 of gestation, and 100% thereafter. Offspring were then fed either ad libitum (CC, n=15 and UC, n=13) or at a level that reduced body weight to 85% of individual target weight (predicted from 0-12 wk growth trajectory) from 12 to 25 weeks postnatal age and ad libitum thereafter (CU, n=10 and UU, n=13). Each group contained approximately equal numbers of males (n=27) and females (n=24). At 2.5 years cardiac morphology and left ventricular function was determined by transthoracic echocardiography under general anaesthesia (2% halothane in oxygen). Data (mean ± S.E.M.) were analysed by ANOVA and a Bonferroni post-hoc test. In male but not female offspring an increase was seen in the interventricular septal wall thickness (UC, 11.2 ± 0.3mm vs. CC, 8.5 ± 0.5mm, p<0.01) in the early gestation undernutrition group (UC) compared to the control group (CC), and also in the mean left ventricular wall thickness (UC, 10.4 ± 0.5mm vs. CC, 8.5 ± 0.4mm, p<0.05). This effect was not seen with exposure to both periods of undernutrition (UU). These results were independent of blood pressure. No changes seen in left ventricular fractional shortening. This study suggests that the increase in left ventricular wall thickness following early gestation undernutrition is sex specific. Mismatches between the in utero and postnatal environment may have important consequences for adult cardiac morphology. The absence of this phenomenon in the female adult offspring may reflect different growth rates and susceptibility between the sexes in early gestation.