The National Aquarium in Baltimore has set a half-price day on Friday, Feb. 5, the second of four half-price days the facility offers each year.

On that day, the $4.50 general admission price for adults will be reduced to $2.25, the cost for senior citizens and students will drop from $3.50 to $1.75, and children 3 to 12 years old will be charged $1.25 instead of the usual $2.50. Children under 3 always are free.

The aquarium is located on Pier 3 of Baltimore's Inner Harbor. It is open from 10 a.m. to 6 p.m. daily except for Friday, when its hours are 10 a.m. to 9 p.m. Ticket sales stop one hour before closing.

The aquarium also offers annual memberships. Members are permitted free admission for a one-year period, a 10 percent discount in the gift shop and a one year's subscription to the aquarium's magazine. Members and up to two guests can enter the aquarium through a separate door, avoiding any lines.

The cost of a membership is $40 for a family (with children up to 18 years old), $15 for college and graduate students, $10 for grade school through high school students, $20 for an individual, $30 for a couple, $15 for a single senior citizen or a couple and $15 for an active military person and his family.

Applications for membership are available by calling (301) 727-3000, ext. 241; by writing the National Aquarium, Pier 3, 501 E. Pratt St., Baltimore, Md. 21202; or at the door.