In searching for effective malaria chemosuppressives during pregnancy in Muheza District--Tanzania, pregnant women are randomly given either 300 mg base chloroquine once weekly or 200 mg daily proguanil. Breakthroughs presenting with clinical malaria are treated with 25 mg base chloroquine/kg (25 CQ) over three days. Due to loss of malaria immunity during pregnancy and Muheza moderate levels and degrees of chloroquine resistance, the in vivo response to 25 CQ was monitored. Between March and May 1989, 49 women were treated resulting into 32 (65%) parasitological clearances and 17 (35%) failures within 7 days. Two of 17 failures (12%) exhibited RIII response and the remaining 15 (88%) had a favourable clinical response. Only 6 (19%) of 32 cleared patients either recrudesced or got reinfected during the three weeks follow up period. In addition to its safety and affordability, the observed drug efficacy during peak malaria transmission and inspite of prevailing resistance makes 25 CQ an ideal first line drug for the management of malaria during pregnancy.