It has been seen for decades as a mecca for explorers and entrepreneurs, a place of sunshine, good times and pleasant retirement, but the West has a darker side: the highest suicide rate in the nation.

Explanations for this curious contradiction have been as numerous as psychiatrists in Beverly Hills and, in sorting them out, researchers have begun to puncture some myths about the causes of suicide, including the possibility that chemicals in the brain may underlie suicidal tendencies.

Among the other conclusions and theories formed in a series of new suicide research projects:

* Heavy migration to the West, bringing a high proportion of disturbed personalities looking desperately for change, may provide an essential clue to the high suicide rate.

* San Francisco, long considered the U.S. suicide capital, may have a high suicide rate not because of its famous bridges and unconventional life styles but because of its small size, large elderly population and an active coroner's office that uncovers suicide as a cause of death in more cases than do other jurisdictions.

* Cultures that discourage public emotion and disapprove of children attending funerals, such as Germany, may have much higher suicide rates than more demonstrative ethnic groups, such as the Irish, and those differences may continue for the first one or two generations in the United States.

* The combination of many of these factors may infect many westerners with what might be called the Meriwether Lewis complex, after the famous explorer of the Louisiana Territory, who lost his father at age 5, tried to work out his emotional troubles by exploring the West and apparently committed suicide at 35.

Suicides do not increase in December, despite suggestions that many people are unusually depressed during the holiday season.

Suicides in the western United States occur at the most recently calculated annual rate of about 17.7 per 100,000 population, compared with 13.8 in the South, 12.3 in the northern and central states and 10 in the Northeast.

Howard Kushner, a San Diego State University historian studying the connection between suicide and migration, said, "When new people get here and find that the change of place does not solve their problems, they may take the next step, suicide. You might look at the West as the next-to-last stop."

Kushner has joined a group of San Diego scholars, including psychologists, sociologists, neuropathologists, epidemiologists, biochemists, pathologists and historians, in planning a massive study of 350 San Diego suicide victims.

The study, if funded, would include what University of South Carolina sociologist and suicide expert Ronald Maris called the most interesting new approach to the suicide issue: brain chemistry.

At the Salk Institute in La Jolla, near San Diego, researchers have devised a new way to detect norepinephrine and serotonin, two chemicals in the brain that appear to influence moods in humans.

John Morrison, a neurobiologist at the institute, said experiments in Sweden have shown a decrease of serotonin in the cerebral spinal fluid of patients who later committed suicide, but so far it has been difficult to detect such chemicals in the brain.

Maris, past president of the American Association of Suicidology, said chemical studies may offer the first chance in some time for an improvement in the treatment of potential suicide victims.

Suicide prevention centers in Los Angeles and San Francisco have helped stimulate an interest in the subject, and may also have led to improvements in identifying suicides that has resulted in an increase in the reported rate, Maris said.

Richard Seiden, a suicide expert at the University of California at Berkeley, said his research indicates that the higher suicide statistics here accurately reflect a higher proportion of westerners taking their lives. But he added that the high suicide rate in San Francisco, now about 27.5 per 100,000 population, may have been augmented somewhat by an active coroner's office.

In fiscal year 1981, San Francisco medical examiners performed autopsies on all but one of the 1,815 cases referred to them, or 22 percent of the city's 8,300 deaths in that period.

In Washington, D.C., where the suicide rate was only 9.1 per 100,000 population in calendar year 1980, the medical examiner's office performed autopsies on only about a third of the 3,020 cases referred to it, or about 15 percent of the city's 6,982 deaths.

San Francisco, unlike Washington, Los Angeles and the nation as a whole, reports that drugs are overtaking firearms as the most popular method of suicide, another indication to Seiden of careful pathological work.

In his research, Seiden said, he was able to rule out the presence of San Francisco's many bridges (cause of only about 12 percent of the city's suicides) and its cosmopolitan culture as a cause of high suicide rates.

One factor, he said, may be the high proportion of unmarried individuals in the city, but also significant is San Francisco's small size, giving it room only for densely populated urban neighborhoods, and its high percentage of elderly.

Heavy urbanization tends to raise suicide rates, and people over 65 are known to have the highest suicide rates throughout the country, with the under-24 age group having the lowest rates.

Kushner, in advancing his theory of immigration feeding suicide rates, argues that moving to a new country or state provides the suicide-prone with a way to resolve their problems. Although the suicide-prone are a small minority of immigrants, they tend to congregate in attractive areas like the West and thus raise the suicide rate, both now and in the late 19th century, which Kushner has been studying.

The West, Kushner said, has been "the least structured" part of the country in family and social apparatus. At least in the past, "it was possible for an Irish immigrant to go to some neighborhoods in New York and somewhat duplicate living in Ireland. It's hard to do that in the West," he said.

Statisticians caution against comparing suicide rates from country to country because of different national systems for collecting the data. But Kushner and others see a significance in the wide differences in the suicide rate for countries like Ireland (about 9.7 per 100,000 for males 15 years old and over) and Greece (4.6), compared with West Germany (35.8) and Austria (47.4).

Kushner suggests that the emotional Irish wake in which everyone, including children, participates may help people work out their feelings about death and prevent future suicides.

Germanic cultures do not encourage such rituals, Kushner said. He said a German woman once told him, "We Germans were shocked when Jackie Kennedy brought her children to President Kennedy's funeral."

Maris said countries like Germany and Austria also may have high suicide rates in part because they have a higher proportion of elderly people.

Freudian psychoanalysts have often theorized that early loss of a parent may create feelings that lead to suicide later in life, although Maris, in a study of 266 suicides in Cook County, Ill., said he found that to be an insignificant factor. More important, affecting about 12 percent of the cases, was a previous suicide by some member of the victim's immediate family.

A nationwide random sample of suicides, with extensive research on their physical condition and emotional history, might provide enough data to settle many of these controversies , Maris said, but such a study "is probably too expensive."