THE FIRST MAN IN ROME By Colleen McCullough Morrow. 896 pp. $22.95

THIS GARGANTUAN novel is splendid in conception and peculiar in execution. Colleen McCullough's research, painfully thorough, not only provides the underpinning for this tale of intrigue and bloodlust set in 110 B.C. Rome, it actually gets in the way. Oftentimes research is the story.

For example, an early dinner scene, wherein Gaius Marius, a rich soldier and outsider, sups with Gaius Julius Caesar Nepos, a not-so-rich blueblood, is a straight version of "Trimalchio's Feast," that rollicking sendup of Roman table habits by the incomparable Petronius. An exhausting list of roast birds, baby vegetables, and other edibles fills up half the page. While such detail is admirable for a classics student, it detracts, ultimately, from the drive of the story.

One rummages through history to find a better story than the rise of Rome as the premier international power. The period of The First Man in Rome covers the beginning of the end of the Republic, a time that became even more golden the further away from it the Romans moved. This period was also used as a standard of exellence during political campaigns, a tactic still much in use today. The truth, and this is vividly captured by McCullough, is that the Republic was as filled with venal men, crass women and irresponsible politicians as any other period of history. What makes this time so compelling is the personalities of the men and women who are in conflict, not simply over money and self-aggrandizement, but over the future of Rome.

The characters of Gaius Marius, Lucius Cornelius Sulla, Julia, her quasi-fictional sister Julilla and Jugurtha, King of Numidia, are well conceived yet burdened with dialogue not only un-Roman but nearly unreadable. Consider this speech by Sulla to the senior Caesar, as Sulla asks to marry Caesar's youngest daughter, the beautiful, doomed Julilla: "When a man can espouse the life his birth entitles him to, Gaius Julius, there is no need for wild parties or dubious friends . . . They were a way of passing the time. I daresay that must seem inexplicable to you. But the life I have lived for over thirty years has hung on my neck like a huge millstone." While a request for a lady's hand in marriage does not necessarily involve passion, surely the language need not be this turgid.

What a pity, because the narrative sweeps along as does the force of history. McCullough's gift shows to good effect the relationship of King Jugurtha to his half-brother Bomilcar. Jugurtha, treated as a cast-off client of Rome while his kingdom is being chewed over by lesser jackals, relies on his half-brother, whose mother was low-born and of another tribe. Both men, creatures of their backgrounds, recoil at the hypocrisy and greed that is Roman politics. The development of this relationship against the backdrop of assassination and war proves that Colleen McCullough understands the undercurrents of human emotion. Bomilcar betrays his king and brother not so much for power, but because his brother never claimed him as kin and never treated him with respect, yet relied on him heavily and burdened him with crushing tasks. The gruesome denouement, in a torture chamber, between these two men redeemed the novel for this reviewer, as did the last days of Julilla.

Love in its various guises fascinates McCullough, which is a good thing because it appears to fascinate the rest of us as well. The rise of Sulla, a gorgeous wastrel, and the fall of his wife, Julilla, again show this author at her best. She understands the bloodknot between this man and woman, so ill-suited for one another and yet incapable of untying their bonds. He needs her for his rise to power and she needs him because, unfortunately, she loves him. When she confronts her husband with a long-term infidelity with a German woman (Julilla has endured his short-term infidelities) he replies about the barbarian, "She never expected me to be what I'm not . . . She belonged to herself, and so she didn't burden me with herself."

Few women writers could have written that passage. Few could have reached into the mind of a male character and revealed female expectations, female demands and the tyranny of female independence.

In this respect, McCullough has always been exceptional. She doesn't load the dice for men or for women. She reveals people as they are, and in this case she is able to create them freed from a Christian overlay, that sneering intolerance of "pagan" psychology.

This novel, flawed yet worth reading, will surely lead her deeper into the labyrinth of Roman history. One need not be conversant with the period to enjoy the novel. One need only be warned that the ponderous proof of research and the dialogue will slow the enjoyment. Perhaps as the author becomes more comfortable with the period she will find less need to parade what she knows about it and will simply tell the story.

The First Man in Rome invites comparison with Taylor Caldwell's A Pillar of Ironies, set a few years later and with that chatterbox, Cicero, as a central character. There is an effortlessness to Caldwell's work. She was regarded merely as a popular historical novelist and yet in retrospect, one realizes how very, very good a writer she was. This reviewer hopes that Colleen McCullough will press on with her saga of Rome and that she will take a lesson from Taylor Caldwell. Rita Mae Brown's most recent work is a mystery, "Wish You Were Here."