Douglas Tims, a 30-year-old Cleveland, Miss., insurance company executive whose background is wealthy Delta Republican, whose suits are three-piece, and whose hair is short was reading a newspaper back in January when he realized his "case was made."

For more than a year Tims, the father of two young girls, had lobbied state legislators, seeking the removal of criminal penalties against persons caught holding small amounts of marijuana.

In Mississippi his task was considered difficult if not impossible.

The lawmakers had convened in early January and Tims was gearing up for another push when he read an article reporting that four out-of-state men charged with smuggling nine tons of marijuana into Mississippi had pleaded guilty and received $15,000 fines but no jail terms.

Two days later, he read another article reporting that two men convicted of hauling more than seven tons of marijuana into the state were given suspended jail sentences and no fines.

"I was amazed," Tims recalled recently. "There it was. The case was made right there."

By the end of the legislative session, the often stodgy Mississippi legislature had passed a law removing jail terms as a penalty for persons convicted the first time for possessing an ounce or less of marijuana and imposing tough, mandatory sanctions against hard-drug traffickers and possessors of large quantities of marijuana.

If Gov. Cliff Finch signs the bill into law, which his legislative aides say he will soon, Mississippi would become the eighth state to end jail terms for persons convicted at least the first time for simple possession.

Other states which, in varying degrees, have removed some or all criminal sanctions for simple possession are Oregon, Alaska, Maine, California, Colorado, Minnesota and Ohio.

"The fact that Mississippi has taken this action should be a signal to other states that it is realistic and possible to do without a backlash," Peter Meyers, chief legal counsel for the National Organization for Reform of Marijuana Laws (NORML), said after the bill was passed.

"After all," he added, "who wants to be behind Mississippi?"

To Tims, one of the great ironies of the legislation is that it would make it less an offense to possess a small quantity of marijuana in Mississippi than to be caught with a can of beer in a dry county.

As an heir from a prominent Delta family that pioneered the Mississippi Republican PArty (although Tims himself claims no party allegiance), Tims found himself well received by leaders of a legislature dominated by Deltans.

Tims, said House Speaker C. B. (Buddy) Newman, "had a lot of influence on me. He made a lot of sense and more than any one person influenced my position."

Tims gained early support in 1975 from a family friend, state Sen. William B. Alexander of Cleveland, who later was elected president pro tempore of the state Senate.

Acceptance of him by legislative leaders, Tims said, "opened the door for me to present my case. and the case is a strong one."

By including in the measure tough and mandatory minimum sentences for persons convicted of possessing large marijuana quantities or of trafficking in hard drugs, Tims said, "they found a way to reduce small possession penalties and still cover themselves at home."

And in a reference to his conservative appearance, he said, "I think having a guy coming in with relatively short hair and a three-piece suit had something to do with it."

While Tims last year pushed the legislation as head of the Mississippi chapter of NORML, he dropped the connection with it this year.

Following reports that NORML might someday seek reduced penalties for cocaine possession, Tims put his association with NORML into the background and worked for the legislation on his own.

While Mississippi's enactment of the law may surprise outsiders, legislative leaders, corrections officials and the state's top narcotics officials are, for the most part, biase about it all.

The measure, at varying times during the legislative process, picked up support from Kenneth W. Fairly, the chief of the Mississippi Bureau of Narcotics; Dr. Allen Ault, Mississippi's corrections commissioner; Carlton Turner, director of the federal Marijuana Research Project at the University of Mississippi; and the state's police chiefs' association.

Much of the legislative support apparently came from lawmakers whose friends and relatives had encountered the present law, which sets a maximum fine of $1,000 and maximum sentence of one year in county jail.

"The more people who get arrested, the more likely it's got to happen that way," Fairly said.

"When your friends get caught with this stuff, or your relatives or their children, well, it gets really close to home," said House Speaker Newman in explaining the legislative acceptance of the bill.

Newman said he became "convinced that what we had was some little ole fellows caught with a little marijuana and getting the shaft thrown at them and being put into prison with hardened criminals, while the big boys, the pushers, the leeches on society were out walking the streets."

Advocates of reduced simple possession penalties were surprised at much of the support they gained from lawmakers they thought would battle them.

It was nearly a foregone conclusion that Rep. Stone Barefield of Hattiesburg, considered by many the most conservative of all legislators, would fight the bill.

But Barefield became one of the stunchest backers of the measure and even offered an amendment to cut penalties against persons convicted of small "courtesy" marijuana sales.

For Barefield, as with many others, the conversion came when a friend's son was sentenced to the state penitentiary for selling a small amount of marijuana to a person he believed was a friend but who turned out to be a narcotics agent.

Several lawmakers said they decided to vote for the bill after talking with their children. Others admitted privately they had sampled marijuana.

And others noted that marijuana was so widespread in this agricultural-based state that some was discovered growing wild on farmland owned by U.S. Sen. James O. Eastland (D-Miss.) several years ago.