It follows the seizure of €140,000 of drugs posted to an address in Lettekenny yesterday.
In a follow-up operation a 28 year old man was arrested and is being questioned at Letterkenny Garda station.
It’s understood the courier company DHL was being used to send drugs into the county.
The combined drugs, 4- Methylethcathinone, with an estimated street value of €21o,000, were seized as part of the inquiry.
4 MEC, is a synthetic stimulant that is chemically similar to Mephedrone.
Gardai across Donegal have been carrying out numerous searches as part of an overall campiagn against drug dealers.
Officers raided a house in the Mountaintop area as well as fields outside the town.
The drugs seizures are being seen as a major breakthrough by Gardai.
A man is expected to appear in court later.