The stratosphere-troposphere (ST) mode operation of the Indian mesosphere-stratosphere-troposphere (MST) radar provided evidence of multiple stable layer structures near the tropopause and also of its weakening during late night hours on several occasions. Mass exchange can take place between the troposphere and the stratosphere during periods of such weakening. To examine whether there is any transport of ozone from the stratosphere to the troposphere at the time of tropopause weakening at tropical latitudes, simultaneous observations were carried out using the Indian MST radar located at Gadanki (13.5 ¿N, 79.2 ¿E), now fully operational in MST mode, and ozonesonde flights from Trivandrum (8.9 ¿N, 76.6 ¿E). Four campaigns of simultaneous observations were conducted during January 3--8, November 9--16, and December 5--10, 1994, and June 7--24, 1995. The results show occurrence of the tropopause weakening several times during January 3--8, November 9--16, and December 5--10, 1994. Simultaneous observations of ozone profiles, particularly on January 7--8, November 10 and 16, and December 7--8, 1994, the days of tropopause weakening, show evidence of a decrease in stratospheric ozone and a corresponding increase in tropospheric ozone after this event, the total ozone remaining essentially constant on these days with respect to other days. Tropopause weakening on January 4--5, 1994, however, does not show any considerable change in the ozone profiles. The results are presented here relating the degree of tropopause weakening to the extent of vertical mass exchange between the troposphere and the stratosphere. An effort has been made to examine the role of horizontal advection of ozone in increasing tropospheric ozone at the region of interest; the geographical distribution of tropospheric ozone residual (TOR) has been analyzed by the two data sets obtained independently from Meteor 3 total ozone mapping spectrometers (Meteor 3 TOMS) and the Upper Air Research Satellite (UARS). Simultaneously, the possibility of horizontal transport of ozone from the areas surrounding Tirupati and Trivandrum in increasing tropospheric ozone due to synoptic scale circulation has also been verified by close inspection of synoptic weather charts at 1000, 700, 500, and 300 hPa obtained from the National Center for Medium-Range Weather Forecasting general circulation model at 0000 and 1200 UTC on January 7 and 0000 and 1200 UTC on January 8, 1994. ¿ 1998 American Geophysical Union