WASHINGTON (AP) _ The Supreme Court today let stand a ruling that people have no constitutional ly protected right to privacy when at home speaking on a cordless telephone.

The court, without comment, turned away arguments that law enforcement agents acted unlawfully when, without obtaining a court warrant, they monitored an Iowa family's cordless-telephone conversations.

Noting that the technology of cordless phones makes conversations on them more susceptible to interception, a federal judge said, ''Because there was no justifiable expectation of privacy, the interceptions did not violate the Fourth Amendment.''

The Constitution's Fourth Amendment generally bars warrantless police searches and seizures, and since a landmark 1967 Supreme Court decision police have been barred from secretly listening to conversations over traditional phones without court warrants.

The 8th U.S. Circuit Court of Appeals upheld the judge's ruling in the Iowa case last June 15, making it binding law in seven states - Iowa, Arkansas, Minnesota, Missouri, Nebraska, North Dakota and South Dakota.

Today's Supreme Court action does not carry any direct impact on any state aside from those seven.

One of every four American homes has a cordless phone, according to industry estimates. Over nine million of the phones were sold in 1989.

The cordless phone relays a speaker's voice from a mobile handset to a base unit through the use of low-power radio transmissions. From the base unit, the signal is transmitted through regular telephone lines.

The phone can be used anywhere within a certain distance from the base unit, usually a range of up to 700 feet. Conversations on it, however, may be heard by another cordless phone nearby and on the same frequency.

Scott and Sheila Tyler of Dixon, Iowa, received a cordless phone as a gift in June 1983. It was regularly used by the Tylers and their children as another extension phone.

Neighbors Rich and Sandra Berodt, who lived several blocks away, soon discovered that their cordless phone would make ringing and dialing noises whenever the Tyler phone was in use.

Based on some conversations they listened to, the Berodts suspected that Scott Tyler was involved in illegal drug dealing. They contacted Everett Howard, an investigator with the Scott County Sheriff's Department.

Howard encouraged the Berodts to continue the eavesdropping, and supplied them with recording equipment so conversations on the Tylers' phone could be preserved on cassettes.

The Berodts' suspicion about drug-dealing proved unfounded, but over several months authorities relied on the tapes to charge Scott Tyler with criminal theft and conspiracy. The tapes, however, were barred as evidence at his criminal trial.

Tyler was convicted in late 1984 on charges stemming from his bilking two food companies out of $35,000. He was sentenced to 10 years in prison, but was released after serving 123 days.

Tyler and his family later sued the Berodts, Scott County, Sheriff Forest Ashcraft and sheriff's investigator Howard in federal court.

The lawsuit contended that the warrantless monitoring of the Tyler cordless phone violated the family's Fourth Amendment privacy rights.

U.S. District Judge Harold D. Vietor in Davenport threw out the suit, and the dismissal was upheld by the 8th Circuit court.

In the appeal acted on today, lawyers for the Tylers said the technological differences between a cordless phone and a traditional phone are not enough ''to remove all Fourth Amendment protection for the daily communications of millions of people.''

''Americans justifiably expect privacy in their residential phone conversations,'' the appeal said. ''That expectation is not based on faith in technology but on faith in our constitutional system.''

The case is Tyler vs. Berodt, 89-691.