Sens. Alan Cranston (D-Calif.) and Arlen Specter (R-Pa.), both up for reelection in 1986, spent $1.6 million and $789,189 respectively on mass mailings for just one quarter of the year, according to a report issued yesterday.

If these spending rates are maintained for the full year, Cranston would spend a total of $6.5 million and Specter, a total of $3.2 million.

Ten other senators were spending at annualized rates in excess of $1 million. They are: Bill Bradley (D-N.J.), $1.4 million; Alfonse M. D'Amato (R-N.Y.), $1.2 million; Jeremiah Denton (R-Ala), $1.8 million; Alan J. Dixon (D-Ill.), $1.3 million; Christopher J. Dodd (D-Conn.), $2.4 million; Charles E. Grassley (R.-Iowa), $2 million; John Heinz (R-Pa.), $2.6 million; Daniel Patrick Moynihan (D-N.Y.), $1.1 million; Donald W. Riegle Jr. (D-Mich.), $2 million; and Paul Simon (D-Ill.), $1 million.

These calculations are based on figures from July through September, in a non-election year. According to estimates by the Senate Rules Committee, spending for mass mail is expected to sharply increase in the 1986 election year.

The dollar figures present a somewhat distorted picture because they fail to reflect the high per capita spending of many small-state senators. On a per capita basis, the highest-spending senators are Dodd, Grassley, Patrick J. Leahy (D-Vt.), Mark Andrews (R-N.D.), James Abdnor (R-S.D.) and Steve Symms (R-Idaho).

For example, Andrews and Abdnor reported spending $81,176 and $82,924 respectively for the three-month period. On a per capita basis, the two were spending at the rate of 12 cents a resident, compared to Cranston's 6.7 cents. If South Dakota and North Dakota were as populous as California, the two would each be spending at annual rates in excess of $10 million.

The mailing figures were publicly disclosed by the Secretary of the Senate for the first time yesterday, largely as a result of pressure from Sen. Charles McC. Mathias Jr. (R-Md.), chairman of the Rules Committee.

Mathias said that public disclosure will remind senators that "mailings are not cost-free and that the real control of this rests with the voters." The House has no such disclosure rules.

A number of senators or their aides issued statements contending that the mailings are an essential method of communicating with constituents.

Beverly Hubble, an aide to Grassley, said the mail costs "are a pretty accurate reflection of priorities here." She argued that Grassley, a leading critic of Pentagon spending, goes home to special town meetings almost every weekend, and he uses the mailings to alert constituents to the sessions.

She argued that the three-month period cited by the report is probably one of the highest for the year, because it included the August recess, when Grassley held a high percentage of his town meetings.

Similarly, Paul Michel, Specter's administrative assistant, said that "more than two-thirds of these mailings related to 26 town meetings the senator conducted" during the summer.

Cranston, acknowledging that his spending "is easy to get excited about," pointed out that he ranks "14th in the Senate on per constituent mailing costs." The 13 senators ranking above Cranston on a per constituent basis, "spent a combined total of $3,084,827 . . . nearly twice as much money as I to communicate with roughly the same number of constituents."

Locally, Mathias reported spending $133,180 for the quarter; Sen. Paul S. Sarbanes (D-Md.), $123,091; Sen. John W. Warner (R-Va.), no expenditures; and Sen. Paul S. Trible Jr. (R-Va.), $70,572. CAPTION: Picture, Sen. Alan Cranston . . . spent $1.6 million in 3 months.