The first part of the fly-on-the-wall documentary series about the women and young offenders' prison Holloway focused on a teenage girl who regularly floods her cell and tries to self-harm, testing the patience of prison officers whose job is to stop her from killing herself.
It won the prime time slot with a 17.9% share of the 9pm to 10pm audience.
Five's 'CSI: Crime Scene Investigation' came in second in the ratings battle with 3.9m viewers (16.6%) tuning in to see Gil Grissom, played by William Peterson, solve his final case.
The penultimate episode of the current series of BBC One drama 'Mistresses', starring Sarah Parish and Sharon Small, pulled in 3.8m viewers and a 16.4% share of the audience.
Last night's instalment saw Jess taking the advice of a relationship self-help guru and Katie accompanying her boyfriend to Australia leaving her boss and ex-lover behind.
'Heston's Tudor Feast', this week featuring frog-meat blancmange, an exploding "cockentrice" and rice pudding made with bone marrow, netted 2.3m viewers and a 9.8% share of the 9pm to 10pm audience. Including Channel 4+1 figures it reached 2.6m viewers.
Heston Blumenthal was at home with the Tudors who also liked mixing sweet and savoury, and meat and pudding.
BBC Two's 'Horizon: Who do you want your child to be?' documentary, presented by comedian David Baddiel whose father told him doing arts subjects at A-level was a "waste of a brain", scored 1.3m viewers and a 5.6% share of the audience.
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