Only 5 of the total of 42 foals born in the semi-reserves died during the past 7 years: two being fully formed stillborn foals and three died, their deathcauses being septicaemia and polyarthritis. The selection pressure on the zoo born stallions in the semi-reserves was much stronger. Horses with congenital diseases soon died in the semi-reserves. Two zoo-born stallions were sent to be National Parc des CÚvennes for testing out a new territory offered by the Parc. One stallion showing symptoms of ataxia broke his shoulder in the rather rough area. The other was struck by lightning. In the Dutch semi-reserves three stallions showed symptoms of ataxia. One highly inbred stallion died of infection of parasites in spite of the normal prophylactic treatment; his resistance was minimal and his inbreeding coefficient extremely high (F. 520). It is evident that weak Przewalski horses and horses showing diseases will be selected out sooner in semi-reserves.