Abstract: In view of the fact the resistance of Plasmodium falciparum to chloroquine occurred extensively in Hainan,a decision was me.de in 1979 that the use of chloroquine should be quitted in the whole province.A longitudinal survey on chloroquine-sensitivity of P.falciparum was carried out during 1981-1991 to observe the variation in resistance of the parasite after the cessation of the chloroquine medication for every 2-3 years.A tendency of progressive decline of resistance was revealed.By using in vitor test,the rate of chioro-quine-resistant P.falciparum dropped from 97.9% in 1981 to 60.9% in 1991 (P0.001).The mean dosage of chloroquine for complete inhibition of schizont formation declined from 10.46pmol/μl in 1981 to 3.02 pmol/μl in 1991 (P0.001).The percentage of population requiring larger dosage (6.4pmol/μl to completely inhibit schizont formation declined from 83.3% in 1981 to 17.4% in 1991 (P0.001); whereas those requiring small dosage (1.6 pmol/μl),increased from 4.2% in 1981 to 60.8% in 1991 (P0.001).In in vivo test,the rate of chloroquine-resistant P.falciparum decreased from 84.2% in 1981 to 40% in 1991 (P0.001).The proportion of RⅡ plus RⅢ cases of the total resistant cases dropped from 59.4% in 1981 to 37.5% in 1991 (0.02P0.01).