We have previously suggested that the recognition of a cross‐reactive epitope on the 5‐HT₄ receptor and the 52‐kDa SSA / Ro protein by serotonin‐antagonizing autoantibodies could explain the electrophysiological symptoms of congenital heart block in neonatal lupus. To confirm this hypothesis, we immunized female mice with four synthetic peptides corresponding to the recognized epitopes. All mice developed anti‐peptide antibodies, which cross‐reacted with the Ro52 and 5‐HT₄ receptor peptides and recognized both cognate proteins. Peptide‐immune mice were mated. The pups from mice immunized with the Ro52 peptides had no symptoms of neonatal lupus apart from bradycardia. However, pups from mice immunized with the 5‐HT₄ receptor peptides and bradycardia, atrioventricular block of type I or II, longer QT intervals, skin rashes and neuromotoric problems. The 5‐HT₄ receptor was detectable in the different fetal tissues affected (heart, skin and brain) by immunohistochemistry. Hearts from diseased pups were less developed and showed disorganized myocardial hyperplasia, compared to the normal littermates. These results demonstrate that the serotoninergic 5‐HT₄ receptor is the antigenic target of physiopathological autoantibodies in neonatal lupus.