A tractor-trailer carrying a cargo of toilet bowls overturned on the outer loop of the Capital Beltway at the Rte. 270 juncture near Rockville last night, spilling gasoline and oil onto the roadway and backing up traffic for several miles for more than an hour, Maryland State Police reported.

Police said the truck driver, a 30-year-old Ohio man, was slightly hurt in the 7 p.m. incident but did not require medical treatment. No other vehicles were involved, they said.

The accident, which tossed the huge rig on its right side and ripped open the top of the trailer, was apparently caused by a broken suspension spring near the right front wheel, police said.

Police investigators said the cargo--bathroom commodes in cardboard boxes--remained inside the trailer, but the fuel spill caused police to close all four lanes of the outer loop and a portion of Rte. 270 for a time as crews worked to clear the site.

One lane of Rte. 270 was reopened almost immediately after the accident, and one lane in the beltway's outer loop was cleared at 8 p.m., police said. Rte. 270 was completely passable by 9:15 p.m., but several hours after the accident, the truck was still blocking two lanes as workmen attempted to right it.