The measures were approved last month over the objection of Gov. Rick Perry, who said they would create a logjam for visitors.
Texas Department of Public Safety spokesman Tom Vinger said the detectors were in use Friday.
DPS says lawmakers and staff won't have to go through the detectors if they have building access badges.
The agency says people with concealed-weapon permits or off-duty law enforcement officers with credentials will be allowed to walk around the detectors.
