The Ventra Web site may be unavailable or have limited functionality in the early-morning hours of three days this week, the CTA said.
VentraChicago.com will undergo maintenance from midnight to 2 a.m. on Thursday, Friday and Saturday.
"VentraChicago.com continues to make improvements and updates," CTA spokeswoman Lambrini Lukidis said.
CTA riders have reported to RedEye troubles with functions on VentraChicago.com including balance transfer. Others said they did not receive e-mails with usernames to log onto VentraChicago.com to manage their account.