A man and a woman have been arrested on suspicion of murder after a two-year-old boy died in hospital.
The child was found unresponsive by paramedics at a house on Woodville Road in Birkenhead at about 14:30 GMT on Wednesday.
He was taken to Arrowe Park Hospital but was pronounced dead a short time later, Merseyside Police said.
The 28-year-old man and 24-year-old woman from Birkenhead are being questioned over his death.
The woman was also arrested on suspicion of child neglect.
A police spokeswoman said two other children who lived in the house "have been medically assessed" and placed in the care of the local council.
She said the force had "implemented the protocols for a sudden unexpected death in infancy and a detailed examination of the circumstances is being carried out".