Fetal heart rate (FHR) was monitored immediately before cordocentesis on 105 occasions in 49 red cell-isoimmunized patients at 27–37 weeks of gestation. The fetal hemoglobin concentration, oxygen tension and oxygen content were measured and the values related to the FHR patterns, which were classified as reactive, non-reactive, decelerative (isolated or repetitive), terminal or sinusoidal. In 88% of non-anemic fetuses the FHR pattern was reactive. Extreme deficits in fetal hemoglobin concentration or oxygen content (> 9 standard deviations below the appropriate normal mean for gestation) were consistently associated with sinusoidal and repetitive decelerative patterns. However, with lesser degrees of fetal compromise the relationship between the various FHR patterns and fetal anemia or hypoxemia was less clearly defined.