Health secretary Nicola Sturgeon has faced calls to tackle the plight of thousands of Scots without an NHS dentist.
Orkney, Grampian and Caithness were among the hotspots identified by MSPs in Parliament during Question Time.
Ms Sturgeon admitted the number of people registered in some areas was "unacceptably low".
SNP Aberdeen North MSP Brian Adam said there was a wide discrepancy in the number of people registered nationwide from 38% in Grampian to 71% in Glasgow.
He urged Ms Sturgeon to call on the British Dental Association to encourage members to return to providing NHS services across Scotland - not just in some areas.
Ms Sturgeon said registrations were at a record high across Scotland.
But she admitted: "Some areas of the country have registration rates that are unacceptably low. NHS in the Grampian area would be included in that category. Work is ongoing to improve the rates of registration."
But she said six new practices are set to open in Grampian and ministers have provided £6 million to the area.
BDA claims that it would need £600 million to provide greater access to NHS dentistry - £245 million more than they currently get - was raised by Tory Mary Scanlon. Ms Sturgeon said: "I wasn't entirely clear what the basis of their calculation was."
Orkney MSP Liam McArthur said the situation on the islands has been described as a "crisis" by local health chiefs. He said the loss of three dentists had resulted in 4,00 people being added to the existing 1,500 already on the list to register with a dentist - out of a total population 20,000.