Fyodor Kulakov, at 60, the third youngest member of the Soviet Union's ruling Politburo and tagged by some Kremlinologists here as a potential successor to Leonid Brezhnev, died of a heart attack yesterday, the government announced.

The death emphasized anew the elderly nature of the leadership under the 71-year-old Brezhnev, who is infirm health. The average age of the Politburo is 67. The death takes from the 14-member secretive cirle one of four so-called "new generation" younger men who elevated to it in 1971.

It also underscores the uncertainty of outsiders in trying to guess at the future leadership of the country. No Westerner here has a clear notion of the structure of power within the Politburo, which sets the policy for the country.

Kulakov, was thought to be an especially important member because he sat not only in the Politburo, but also in the smaller central Committee Secretariat, which has direct daily administrative control over the party.

He ran Soviet agriculture and despite its continued troubles, apparently maintained a strong leadership position as a Brezhnev favorite. He represented the Politburo at three recent Communist Party congresses in neighboring socialist states, which was seen here as a sign that he was being groomed for wider responsibilities, perhaps leading to succession.

Barring other deaths within the aging leadership, Kulakov's demise seems likely to result in the most significant realignment of the Politburo since May 1977, when then-president Nikolai Podgorny was forced from office and expelled from the leadership group after losing a fight over Brezhnev's determination to have himself named chief of state as well as general secretary of the Communist Party.

Kremlin watchers here, however, could not predict the timing or the scope of the realignment.

"All we can do is wait for some signs here and there," said one source. "But even knowing how to read them is going to be a problem. We won't know who his replacement is until he shows up one day at an official function, it seems,"

Kulakov's death comes just as the leadership is seeking ways to improve agricultural performance. A much smaller harvest last year than anticipated, has forced the country to spend hard currency buying feedgrains.

It has been the view of several observers here who keep close track of the arcane signs of how power flows within the leadership that possible successors to Brezhnev include Andrei Kirilenko, 70, and party theoretician Mikhail Suslov, 75, and that Kulakov, despite his relative short time within the Politburo, had been a strong prospect eventually to succeed them.

Two other aging men, Premier Alexei Kosygin, 74, and Foreign Minister Andrei Gromyko, who is 69 today have long stood within the inner government circles, although Gromyko was admitted to the Politburo only in April 1973, and his position as foreign minister does not give him much influence in strictly party matters.

Of the 13 remaining Politburo members, five are 70 or more, seven are in there 60s, and one, Leningrad party chief Grigori Romanov, is 55.

According to his official biography, Kulakov was born Feb. 4, 1918, into a peasant family in the village of Fitizh, in the Kursk region of Russia, south of Moscow. A trained agronomist, he joined the party in 1940. His first important party job was as first secretary of Stavropol territory, a major farm region, in 1960.

Kulakov was a supporter of the late Nikita Khrushchev's agriculture reforms. He was brought to Moscow after Khrushchev's ouster in 1964 to head the party agricultural department. He became a central committee secretary the following year and helped improve Soviet agriculture - a perennial weak spot in the economy - by relaxing curbs in private plots, increasing farm investment and devising new state procurement prices for produce.

Kulakov, a powerfully built man who spoke Russian with slurred southern accents similar to Brezhnev, was, in recent years, frequently seen at Brezhnev's right in public appearances of the Politburo. To Western observers this was a sign that despite the ups and downs of agriculture, he ramained a favorite of the party boss.

Like many of the inner cirle, Kulakov was little known in the West and seldom met Western diplomats here. He had been to the West just once, in 1959 as part of an agricultural visit to Britain. He represented the Soviet Union at party congresses in Bulgaria and Mongolia in 1976. This year he represented the Soviet party at the Yugoslavia party congress.

On his 60th birthday, he was made a hero of socialist labor, one of the highest Soviet awards, and a book of his speeches was published.

On July 3, in a lengthy address issued under Brezhnev's name, the country's continuing agricultural problems were blamed in part on a failure by regional party workers and chairmen to fulfill the plan set by the central government. Kulakov seemed unscathed in the unusual attack by Brezhnev, who has been careful throughout his nearly 14 years as leader not to repeat his predecessor Khrushchev's mistake of angering regional party chiefs.

Tass, the official Soviet news agency, disclosed yesterday that Kulakov had suffered from arteriosclerosis, cardiac sclerosis and "chronic pneumonia." It said he had major stomach surgery 10 years ago, but did bot specify the reason.

Kullakov will be buried in the Kremlin wall, a usual mark of respect for a valued party man.