Gisele Bundchen -- 4-Wheeling with Baby Daughter ... Sans Helmets

Safety first ... is not exactly a priority for Gisele Bundchen, who took her 1-year-old daughter on an ATV ride yesterday in Costa Rica -- without a helmet.

Gisele wasn't doing wheelies or anything, but it's still pretty dangerous, considering she was driving one-handed and holding on to her baby daughter Vivian Lake with the other.

For the record, it's illegal to ride an ATV on public land in Costa Rica without a helmet. Gisele's driving down to the beach, but it's unclear if the beach is private.