A federal judge yesterday reversed his own ruling of last May that criticized CIA Director William J. Casey's conduct in a past business venture, saying that Casey's arguments for reconsideration of the earlier judgment were persuasive.

The ruling by U.S. District Court Judge Charles E. Stewart Jr. in Manhattan was entered in a civil suit alleging that Casey and his partners misled investors in a corporate farming venture in the late 1960s. The effect of the judge's ruling is to leave it to a jury to decide whether Casey participated in alleged misrepresentations about the condition and indebtedness of seven farm properties owned by the agribusiness concern, Multiponics Inc.

The ruling was largely directed at exempting Casey from the judge's earlier finding and let stand the adverse judgment against four of Casey's partners in the deal. Petitions filed by Casey's partners also seeking reconsideration were not pursuasive, the judge ruled.

Stewart said that even though it was alleged that Casey was the largest investor in the company, served as corporate secretary and legal counsel, a jury should determine whether he was actively managing the company's affairs and was involved in the alleged misrepresentations.

Stewart ruled on May 19 that the officers and directors of Multiponics "omitted and misrepresented facts" in a stock offering circular seeking $3.5 million in investor funds.

The ruling would have paved the way for damages to be paid to the investors who filed the lawsuit against Casey and his partners in 1975. Multiponics entered bankruptcy in 1971 and is now defunct. The firm had acquired large farm tracts in Florida, Mississippi, Louisiana and Arkansas.

Under yesterday's ruling, Casey alone among the defendants may escape paying damages if he can convince a jury that he did not participate in preparing investor circulars that contained the alleged misrepresentations and, therefore, did not violate the antifraud provisions of the securities act.

In the 20-page ruling, Stewart drew a distinction between Casey and the other defendants. "The case of defendant Casey, who alone alleges an 'outside' position on the Multiponics board, must be considered apart from the other . . . defendants."

Stewart pointed out that a judge must not issue a summary judgment ruling like the one he entered in May where there are any disputed facts and the evidence should be viewed in a light most favorable to the defendant.

" . . . We conclude that defendant Casey's most recent submissions raise an issue of fact as to his status on the Multiponics board. Until the preliminary factual issue of Casey's involvement in the management of Multiponics is resolved, this court cannot determine whether the recklessness previously found by this court should suffice for his liability . . . . "

News reports about the initial Multiponics decision surfaced last summer at about the time that Casey's handpicked clandestine service chief at the CIA, Max Hugel, was forced to resign over allegations that he had engaged in improper stock market practices during the mid-1970s.

The allegations against Casey in the Multiponics case were seized upon by members of the Senate Intelligence Committee already miffed over Casey's judgment in appointing the inexperienced Hugel to the sensitive CIA post.

Intelligence Committee Chairman Sen. Barry Goldwater (R-Ariz.) ordered an investigation of Casey's business dealings partly as a result of Stewart's ruling in the Multiponics litigation, the existence of which Casey had not fully disclosed to the Senate during confirmation proceedings.

Two weeks ago, Goldwater said that the three-month investigation had not turned up anything that called into question Casey's fitness to continue in office. Sources close to the investigation said that the Multiponics case, because it represented conduct more than a decade in the past and because Casey apparently was not the key operating executive of the firm, did not turn out to be the principle focus of the Senate inquiry.

A full report of the Senate inquiry, which one senator has said may include some critical language about Casey's past conduct, is expected to be made public later this month.