The Junior World Cycling Championships will end today with the fourman team trials along the Maryland side of the George Washington parkway. Starting time for the 45-miler is 10:30 a.m.

Twenty-one nations have entered squads in the event, the seventh of the championships which began last week Trexlertown, Pa. Thus far, the Soviet Union has won three gold medals and East Germany, which won the team trials last year, two.

Teams will be riding against the clock as they make four tours around the 11.25-mile circuit, starting and finishing at the Cabin John exit. A draw was held late yesterday to determine the order of departure for all but the top six nations from last year's trials. Those six will leave last, in reverse order of their 1977 finish. The teams will start in two-minute, intervals.

Riders, will swing into single formation immediately, with the lead man doing, most of the work by breaking wind resistance. At specified periods, the lead, will whip his bike out to the left, allow the other three to move up, and fall back into the fourth slot. Teams can pass each other but will not be permitted to draft one another.

Greg LeMond of Reno, Nev., who placed ninth in Sunday's 76-mile road race in Rock Creek Park, leads the United States squad. Joining him will be Jeff Bradley of Davenport, Iowa (16ta), Ron Kiefel of Denver (19th) and Greg Demgen of La Crosse, Wis., who bowed out of Sunday's road race on lap seven with a mechanical problem.

The Soviet Union, which placed three riders in the top 11 on Sunday, is favored along with East Germany, Italy (third in last year's trials) and Switzerland, which had three in the top nine on Sunday.

"The race will be intense from the beginning," commented assistant American Coach Bill Humphries. "Riders will be constantly hammering (pedaling in high gears), which takes a lot out of them in this heat and humidity."