A male newborn infant, studied at autopsy, showed continuous splenogonadal fusion and severe peromelia associated with an unusual, possibly unique, cardiac malformation complex. The cardiac lesions included multiple right ventricular diverticula, tricuspid atresia, mitral to semilunar valve discontinuity, and absent muscular outflow tract septum. Tabulation of the time of appearance of relevant anatomic features in 351 normal human embryos of Carnegie stages 9 through 23 showed that the teratogenic influence in the present case probably occurred by stages 16-17. The nature and distribution of lesions observed suggest that proliferating undifferentiated mesenchyme was the target of the unknown injury.