Two carbamates and one organophosphorus insecticide were evaluated in a village-scale trial in Nigeria by the WHO Insecticide Testing Unit. Toxicological studies on exposed spraymen and villagers included clinical observations, determination of plasma cholinesterase activity and-in the case of the carbamates-determination of metabolites in urine. While a village-scale trial can provide a basis for evaluation of the risk to which villagers are exposed, the risk to the spraymen cannot be fully assessed since the spraying operation in a single village lasts a comparatively short time. The toxicological information gained in this trial has shown that one of the carbamates tested (3-isopropylphenyl N-methylcarbamate) is too toxic to man to be used as a residual insecticide in houses. The other two insecticides (carbaryl and Folithion) have proved safe enough to warrant their being subjected to an extended field trial, provided that precautionary measures similar to those described in the paper are undertaken.