The earlier work of this Institute on phenothiazine in lambs (Peters et al, 1941; J. Leiper et al, 1941) using egg-counts, worm-counts, and lamb-weights as criteria of anthelmintic efficacy, has revealed conflicting results as between one criterion and another. Thus, the lowest dosage rate used (0·15 gm. per Kg.) led to a reduction in the egg-count, whilst even the highest rate (1 gm. per Kg.) led to no reduction in the worm-count: by an irony of chance the worm-counts were lowest in the untreated controls in both experiments. In the first experiment weights were not affected by the drug, and in the second the only significant effect was a set-back in those lambs given tablets. Numerous other workers have reported increased weights due to the drug, in long-term experiments; ours were too short in duration to show such increases, yet the danger of reinfestation precluded any extension of the time, the lambs being kept outside on infective pastures. The most obvious explanation of the egg- and worm-count results appears to be that the drug has a temporary inhibiting effect on egg-laying, which would presumably return to normal in a few weeks; but here again a prolonged experiment increases the liability to reinfestation.

These being the relevant circumstances, the present experiment was designed to take account of them in the following ways. In the first place the lambs were kept inside throughout the experiment, their stalls being cleaned out thoroughly twice a week to obviate reinfestation.