The party's health spokesman, Ross Finnie, said it was the young and the elderly who were targeted when it came to mental health care.
Government figures requested by the MSP show that in nine out of Scotland's 14 NHS Boards, the rate of readmission within 28 days to mental health units has risen since 2007 for those aged 45 to 64 .
The NHS is currently meeting its overall target on reducing mental health re-admissions to hospital. Finnie said: "There are targeted
strategies for young people and the elderly, but there is nothing designed around the needs of those in their 40s, 50s and 60s.
"I hope the government takes immediate action to address this issue."
A spokesman for the Scottish Government said: "We recognise that people of all ages can experience mental health problems. "Many of the priority areas within our mental health improvement strategy, Towards A
Mentally Flourishing Scotland, address the mental health needs of those in middle age.