Great Britain were beaten 30-17 by Brazil, their third successive defeat in their first ever Olympics.
At times, the home side looked dangerous at the Copper Box and caused problems for Brazil, who came fifth in the last World Championships.
Poor finishing and untimely errors proved costly, though, and gave Brazil their third consecutive win.
Lyn Byl top-scored for the second game in a row for GB with four goals, but Brazilian Ana Paula Rodrigues finished with seven.
Britain are next in action on Friday, when they face Angola who were beaten 30-25 by Montenegro also on Wednesday.
In the other Group A match Croatia beat Russia 30-28.
In Group B Norway and Korea drew 27-27, France overcame Sweden 29-17 and Spain won against Denmark 24-21.