A total of about 1.9kg of cannabis and 1.1kg of heroin was seized in the Central Narcotics Bureau's (CNB) first major operation of the year.
The drugs are estimated to be worth more than $142,000 in total, CNB said in a statement yesterday.
The operation took place early on Monday morning near Evans Road in Bukit Timah where CNB officers spotted a 34-year-old Malaysian suspect.
Shortly after, two suspected local drug traffickers - a 46-year-old man and a 30-year-old woman - arrived in a van.
All three suspects were seen walking into a residential estate. The Malaysian man later left on his motorcycle.
CNB officers moved in and arrested the pair near Evans Road.
A check of the woman's locker at her workplace in Evans Road uncovered about 1,933g of cannabis, 665g of heroin and 40g of "Ice" or methamphetamine.
About 26g of cannabis was recovered from the van driven by the man.
He is suspected of having procured the drugs from a drug syndicate based in Malaysia.
The Malaysian man was arrested later on Monday in Woodlands Avenue 3 by CNB officers. Cash totalling $4,500 was recovered from him.
Follow-up investigations led to two more suspects being nabbed.
Two Singaporean men, aged 56 and 62, were arrested at a unit in Jalan Bahagia later that day.
About 461g of heroin was found in the unit, and more than $16,400 in cash was recovered from the two men.
Investigations into the drug activities of all the suspects are ongoing.
Under the Misuse of Drugs Act, those convicted of trafficking more than 500g of cannabis may face the death penalty.
Twice that amount is sufficient to feed the addiction of 143 drug abusers for a week.
Under the Act, those convicted of trafficking more than 15g of diamorphine, or pure heroin, may also face the death penalty.
That amount is equivalent to 1,250 straws, enough to feed the addiction of about 180 abusers for a week.
CNB's director of communications Sng Chern Hong said Singapore "has strict laws for drug trafficking and CNB conducts rigorous enforcement to deter and interdict the flow of drugs into Singapore".
"Drug syndicates - both local and overseas - continue to take their chances to smuggle and distribute drugs to the local market, and reap profits out of the misery of people trapped in the vicious circle of drug abuse," he said.