From 1981 through 1984, the response of Plasmodium falciparum to chloroquine was monitored in Saradidi, Kenya, as a part of a community-based health programme to provide treatment for malaria in each village. Before 1983, all 71 infections treated with chloroquine were sensitive in vivo; parasitaemia cleared by day 3 and remained absent to day 7. In June 1983, 23.1% of 26 infections treated with chloroquine base 10 mg kg-1 either recrudesced in seven days (RI resistance, five infections) or decreased but failed to clear (RII resistance, one infection). In September 1983, 16.2% of 68 and in February 1984, 13.2% of 53 infections were resistant in vivo after treatment with chloroquine base 10 mg kg-1. A course of chloroquine base 25 mg kg-1 over three days remained effective; only two (1.6%) of 129 infections examined were resistant in vivo; in both, parasitaemia cleared then recurred (RI). In September 1984, however, nine (10.2%) infections were resistant after treatment with chloroquine base 25 mg kg-1; in four of these parasitaemia decreased but never cleared (RII). Similar results were observed in vitro. In the Rieckmann macro in vitro test, 63.3% of 30 P. falciparum isolates tested were resistant to chloroquine (minimal inhibitory concentration (MIC) greater than or equal to 1.25 X 10(-6) mol 1(-1) blood) in June 1983, as were 61.8% of 34 isolates in the Rieckmann micro test (MIC greater than or equal to 1.14 X 10(-6) mol 1(-1) blood).(ABSTRACT TRUNCATED AT 250 WORDS)