In 1983 and 1984 the effect of a deworming scheme, in which horses were treated at turn out and 4 and 8 weeks later, on the egg output, larval differentiation, weight gain, herbage infestation and in 1984, the percentages of some serum proteins were tested. In 1983 the experiment was done with 42 mares, 54 two year old male horses and 42 male yearlings, kept in groups with permanent or rotational grazing. In 1984 only 90 male yearlings were investigated. In 1983 albendazole and ivermectin were used, 1984 ivermectin. The results of the faecal examinations showed that after ivermectin treatment the number of eggs per gram faeces (EPG) remained zero or very low for a longer time. In 1983 an increase was seen during August. In 1984 the increase was observed at the end of August/beginning of September. Treatment with albendazole gave a less efficient suppression of egg output, resulting in a sharp EPG-increase at the end of July/beginning of August. After first treatment either with ivermectin or albendazole, no larvae of great Strongyles were seen. The results of the herbage infestation on the paddocks were according to the EPGs of the horses that grazed the paddocks some time earlier. Infestation was highest in late summer and autumn, lowest in early and mid summer. No significant differences in weight gain were observed between the groups. The percentage of beta 1 + 2-globulin was highest at turn out. After first treatment it decreased, but remained stable thereafter during the season, irrespective of the reinfection in autumn. Post-mortem results of four tracers in 1984 varied from ca. 275,000-2,000,000 small Strongyles. The conclusion was drawn that under the present conditions the system was not satisfactory to prevent a reasonable worm infestation in late summer/autumn. The following advice was given: treatment of horses at least 48 hours before turn out, deworming each four weeks up till July, treatment in September and in November (also against Gasterophilus).