ALBANY, May 14— Guy J. Velella, the veteran Bronx Republican, resigned from the State Senate on Friday, ending a tumultuous 31-year career in the Legislature.

Mr. Velella, the head of the Bronx Republican Party, was known in New York politics as a formidable force, dispensing patronage without apology and bringing projects to his district. But his career was also marred by several allegations of ethical lapses.

His resignation stemmed from his most recent brush with the law. He agreed to step down as part of a plea bargain with the Manhattan district attorney's office after being indicted in Manhattan two years ago on charges of taking bribes to help people win lucrative state contracts. His father and law partner, Vincent J. Velella, was also indicted, but as part of the agreement he will not be prosecuted.

Notice of Mr. Velella's resignation came in letters to Lt. Gov. Mary O. Donohue, the president of the Senate, and to Secretary of State Randy Daniels, according to a state official and an aide to Velella, who both spoke on the condition of anonymity. The senator himself declined to be interviewed.

The letters did not give a reason for his resignation, but said it would take effect at midnight Friday.

Mr. Velella is expected to plead guilty to a felony charge in Manhattan Criminal Court on Monday morning and faces as much as a year in jail. He will also lose his law license. It is unclear which charge in the 25-count indictment he has agreed to plead guilty to.

In the indictment, Mr. Velella and his father were accused of taking payments from companies they had represented as lawyers in exchange for helping them obtain state bridge-painting contracts. His departure leaves a seat open in next fall's elections that either a Democrat or a Republican could win. Under state law, there is not enough time for a special election to be held before November, so the seat will remain vacant for the last six weeks of the regular legislative session, cutting the Republican majority's margin in the Senate to 13 members.

Mr. Velella was responsible for pushing through several important insurance law changes. He sponsored legislation mandating coverage for a two-day hospital stay for mothers who have just given birth and forcing insurers to pay for longer hospital stays after mastectomies.

As the chairman of the Senate's Labor Committee, Mr. Velella was also the prime sponsor of a bill to raise the minimum wage this year, and his resignation leaves that piece of legislation up in the air.

But he was also dogged by charges of ethical laxity. During the 1990's, for instance, his small law firm was given hundreds of thousands of dollars in legal work by giants in the insurance industry while he headed the committee that oversaw legislation affecting them.

Photo: Guy J. Velella's resignation ended a 31-year career in the Legislature. (Photo by Stewart Cairns for The New York Times)