We conducted a randomized double-blind placebo-controlled study, in the Ahanta West District of Ghana, on the provocative day test effect of ivermectin and albendazole alone and in combination on nocturnal periodic Wuchereria bancrofti microfilaraemia. Sixty-three individuals with high night-time microfilaria (mf) intensities were identified in 1997 or 1998 and randomized into 4 groups. Blood samples for mf were then collected from the same individuals in the daytime (between 09:00 and 15:00) immediately before and 30-50 min after treatment. Groups 1-4 were treated with ivermectin alone (150-200 micrograms/kg), albendazole alone (400 mg), the combination of ivermectin and albendazole, and placebo, respectively. Intensities of mf in daytime samples were considerably lower than in night-time samples. Neither ivermectin or albendazole alone nor their combination provoked significant liberation of W. bancrofti mf into the peripheral circulation after the daytime treatment.