Two experiments were conducted to investigate the change over time in LD50 values in an in vitro larval development assay. In each experiment, six field-reared Romney lambs were effectively treated with an anthelmintic and housed. In experiment I, six three month old lambs were given a single infection of 35000 infective larvae of T. colubriformis. From ten days post infection (DPI) three lambs (Group 1.1) were treated twice weekly with 0.5mg/kg of dexamethasone trimethylacetate whilst the other three (Group 1.2) served as controls and remained untreated. In experiment II, three lambs (Group 2.2), six month old were infected with a single dose of 22000 infective larvae of T. colubriformis whilst the other three (Group 2.1) of the same age were trickle-infected with 2000 infective larvae once weekly for 14 weeks. Larval development assays were conducted weekly for 14 weeks with ivermectin in Experiment I and ivermectin, avermectinB2 and levamisole in Experiment II. In Experiment I and II for ivermectin, the LD50 values rose to a 4x increase between 50-70 DPIand fell again. The general pattern seen following a single infection with all anthelmintics was for the LD50 values to be relatively constant from 21-35 DPI, then rose 2.5-7x increase to peak 49-56 DPI and declined at the same rate again to original starting values by 84 DPI where they remained until the end of the experiment. In experiments I, the steroid treated group started with similar values but had a 5x fall by 42 DPI which was not seen in the single infection group. The trickle infection group in Experiment II generally resulted in a small increase of 1-1.5x from 42-77 DPI and then declined again to starting values until the remainder of the experiment. The study demonstrates that there is a similar change in the LD50 values with time for T. colubriformis with all three anthelmintics tested and that the change was prevented in steroid treated animals and was less apparent in trickle-infected animals.