David Smith, aged 34, of Torridge Way, Efford, admitted possessing the drug with intent to supply between May 14 and May 17 last year.
David Gittins, prosecuting, told Plymouth Crown Court that Smith had admitted the 'commercial supply of a large quantity of heroin'.
Ali Rafati, for Smith, said he had been working with a drugs caseworker since June last year.
Judge Francis Gilbert ruled that Smith must stay in custody until he is sentenced on March 20. Probation officers are to prepare reports into his background.
He told Smith: "This is a serious offence which is likely to attract a lengthy sentence."