Leaders Olympique Lyon stretched their unbeaten run in Ligue 1 with an exciting 3-2 home win over struggling AC Ajaccio on Sunday.
At the Stade Velodrome, Paris St Germain coach Laurent Fournier was caught up in an ammonia incident and left his side's dressing room coughing before the 1-0 defeat at Olympique Marseille, TV footage showed.
"Actually, both Paris and Marseille's dressing rooms have been cleaned with the same product," said the home team's chairman Pape Diouf.
"We just arrived in the dressing room before the Parisians and had time to ventilate it. However, we went down to the Parisians's dressing room with the chief of police and had it sealed off in case the police want to search it tomorrow."
Canal Plus said ammonia was present in PSG's dressing room before their arrival and Fournier, who felt dizzy, later had his blood pressure checked.
Four-times champions Lyon, who have not lost a match this season, took their points total to 27 from 11 games after squeezing past Ajaccio.
They have opened a seven-point gap over second-placed PSG whose defeat by Marseille was the first to their arch-rivals in three years.
Marseille have moved to seventh with 17 points after winning five of their last six matches.
Brazilian striker Fred opened the scoring for Lyon in the 30th minute after taking advantage of an unintentional back pass by an Ajaccio defender and beating goalkeeper Stephane Porato.
The hosts went back in front on 50 minutes thanks to a Juninho 20-metre drive but the Corsicans levelled again in the 54th minute after a Tiago own goal.
Ajaccio striker Rafik Saifi had a goal disallowed for handball before French international Sylvain Wiltord grabbed a 69th minute winner after a fine one-two with substitute midfielder Karim Benzema.
Marseille dominated at the Stade Velodrome and were rewarded for their efforts in the 78th minute with a goal by former PSG midfielder Lorik Cana.
From a Samir Nasri corner kick, Cana headed home to beat his former teammate Jerome Alonzo.
PSG had a couple of chances through midfielder Sergei Semak and forward Bonaventure Kalou but both were denied by a fantastic Fabien Barthez, returning from a five-month ban.
On Saturday, AJ Auxerre thrashed promoted Troyes 3-0 to move to fourth but Girondins Bordeaux were held to a 1-1 home draw by struggling Sochaux.
Bordeaux, who grabbed their point courtesy of a last gasp equaliser by Argentine midfielder Alejandro Alonso, remain third with 20 points from 11 matches, one point above Auxerre.Sixth-placed Monaco scored their first home goals of the season with a 3-0 defeat of bottom-of-the-table Metz to give new coach Francesco Guidolin a winning start in the principality.