Two Washington-area companies, both reporting substantial growth in recent years, have announced initial public stock offerings.

One of the firms, Insituform East Inc., of Landover, already has begun its offer of 275,000 shares of common stock. The offering is being managed by Ferris & Co. Inc.

Nuclear Support Services Inc. of Woodbridge has filed a preliminary prospectus with the Securities and Exchange Commission disclosing plans to sell 400,000 shares of common.

Insituform hopes to raise more than $2 million in the sale of its stock at $7.50 a share. Proceeds from the offering will be used primarily for capital improvements and purchase of additional equipment.

Insituform is a sublicensee of Insituform of North America, exclusive U.S. licensee for a patented process used to repair underground pipeline.

The Insituform process is a technology in which a polyester fiber felt liner is inserted in a damaged sewer pipe. The liner, which is coated with polyurethane and filled with a liquid, thermal-setting resin, is inserted through an existing manhole or other access point in the pipeline.

Heated water "cures" the resin to form a pipe within the damaged pipe. The process eliminates the need for digging large areas to repair pipelines.

The process was developed and tested in the United Kingdom in 1971. A license was granted for use of the technology in the United States six years later.

A major part of Insituform's business has been its work for the Washington Suburban Sanitary Commission.

The company reported revenues of $4.98 million and net income of $540,065 (54 cents a share) for the year ended June 30 compared with sales of $2.45 million and net income of $313,065 (31 cents) in the previous year.

Nuclear Support Services reported revenues of $12.9 million for the first nine months ended June 30 compared with $7.1 million a year earlier. Net income for the most recent nine-month period was $562,000 (58 cents) compared with $341,000 (41 cents) in the comparable period a year ago.

Assets reported on June 30 totaled $4.34 million.

The company provides a variety of technical services primarily to the nuclear utility industry. Those services include personnel support in maintaining and operating nuclear power plants and during construction and start-up phases.

Proceeds from its offering, which is being managed by Advest Inc., will be used to reduce borrowings under the company's line of credit. The balance will be used to finance an increased level of accounts receivable and to increase working capital.

The initial public offering of NSSI is expected to be between $8.50 and $11.50 a share.

Meanwhile, Minix Ltd., an Annapolis company which specializes in microfilming original medical X-rays and marketing microfilm and related equipment to hospitals, plans to sell 1 million shares of stock at $3 each.

In a preliminary prospectus, Minix said it plans to use the proceeds from the sale of stock primarily to retire debt, acquire equipment and expand markets.

Minix reported a sharp increase in sales for the fiscal year ended June 30 to $1.54 million from $567,140 in the prior fiscal year.