Profits rose in the third quarter at Potomac Electric Power Co. of Washington and Maryland National Corp. of Baltimore, holding company for the largest bank in Maryland, the two firms reported yesterday.

Pepco earned $35 million (79 cents a share) in the recent quarter compared with $26.7 million (64 cents) for the same period last year as revenues rose to $224 million from $173 million.

In the first nine months of 1977, the electric utility's profits totaled $65.7 million ($1.42 a share) on revenues of $513 million compared with earnings of $56.4 million ($1.25) and sales of $419 million in the 1976 period.

Directors yesterday approved regular quarterly dividends on all classes of Pepco stock, with a common stock payout of 32 cents a share payable Dec. 30 to stockholders of record Nov. 29.

Maryland National reported third-quarter operating profits of $5 million (71 cents a share) compared with $4.8 million (68 cents) in the 1976 period. Nine-month profits, not counting securities transactions, rose 21 per cent to $13.7 million ($1.93 a share) compared with $11.3 million ($1.59).

Chairman Robert D. H. Harvey said the higher earnings reflected a combination of loan and deposit growth and improved interest income. Maryland National Bank also received during the recent quarter about $1 million of interest from loans carried on a non-accrual basis, offset by an extra provision of $1 million for potential loan losses.

Deposits averaged $2.1 billion in the 1977 period, up 6.2 per cent from last year, while loan volume increased by $176 million to $1.75 billion.