A controlled study was carried out to determine the effect of some known anti-filarial drugs on the pattern of emergence of Onchoccrca volvulus microfilariae from skin snips into saline. The results show that before treatment, 81·7 to 92·1% of microfilariae emerge in all the groups within 24 hours. The differences between the groups were not significant. After treatment with oral diethylcarbamazine, metrifonate and topical diethylcarbamazine (DEC) the percentage of microfilariae emerging from the snips dropped significantly to 48·4,560 and 60·1 respectively, whereas no significant change occurred following treatment with levamisole (88·1%) and yeast tablets (89·1%). A pair-wise comparison of the post-treatment microfilarial emergence in all the groups showed statistically significant differences when (i) oral DEC is compared to levamisole, topical DEC and yeast, (ii) metrifonate is compared to levamisole and yeast and (iii) topical DEC is compared to yeast. It is suggested that microfilariae which fail to emerge from snips after treatment are those that have been killed or paralysed by the action of the drugs. The study has thus established that drug action in vivo is one of the factors which affects the proportion of microfilariae which emerge from skin snips and provides an additional parameter for measuring the antimicrofilarial potential of drugs in clinical chemotherapeutic trials.