A woman and her two-year-old son were rescued from the River Thames, emergency crews said.
The boy had fallen through open railings into the water when his 34-year-old mother jumped in after him, a spokesman for Chiswick Royal National Lifeboat Institution (RNLI) said.
Both mother and child were taken to hospital following the rescue operation in Isleworth, west London, at about 5pm.
They were described as "very cold" but did not appear to be seriously hurt, London Ambulance said.
A London Ambulance spokeswoman said: "We were called just before 5pm to reports of a mother and child in the river at Bridge Wharf Road, Isleworth.
"We sent an ambulance crew, single responder in a car, a manager and our hazardous area response team (HART) to the scene.
"The first of our staff arrived in just over three minutes of the call coming into our control room.
"HART - a team of specialist medics trained to treat people in difficult circumstances - lowered ropes to the patients who were then helped to safety by the RNLI.
"Both patients, a woman reported to be 34 years old and her two-year-old son, were very cold but did not appear to have any other injuries.
"They were taken to West Middlesex hospital as a priority."
London Fire and Rescue service, London Ambulance, the Metropolitan Police and Chiswick RNLI were involved in the rescue operation.
A spokesman for Chiswick RNLI said: "The London Ambulance Service was nearby and managed to throw a line to the woman before the lifeboat crew arrived seven minutes later from Chiswick and got them out.
"When our crew arrived the baby was crying and mother was able to confirm that no one else had fallen in.
"They both seemed to be fine but they had been in the water 10 minutes and it was eight feet deep, so it was a very hazardous situation."