A 3-month experiment was conducted at a 300 kg scale to observe decomposition processes in pig slurry solids amended with two different doses of natural Slovak zeolite-clinoptilolite (substrates S1 and S2, 1% and 2% of zeolite by weight, respectively) in comparison with the control (unamended solids). The experimental and control substrates were stored outdoors in sheltered static piles at ambient temperatures ranging from 8.0 to 34.7 degrees C. The solid fraction (SF) of pig slurry was obtained by separation on vibration sieves prior to slurry treatment with activated sludge. The initial water content of the SF was 77.1% and no water was added to the piles during the storage. The temperature in the core of the piles was recorded throughout the experiment. By day 3 and 5 of storage (1% and 2% zeolite, resp.), the temperature in the substrates S1 and S2 exceeded 55 degrees C and remained above this level for 15 days while the highest temperature recorded in the control during the experiment was 29.8 degrees C. Samples from the core of the piles were taken periodically to determine pH, dry matter at 105 degrees C (DM), ash (550 degrees C/4 h), ammonia nitrogen (N-NH(4)(+)), nitrate nitrogen (N-NO(3)(-)), total nitrogen (N(t)), total phosphorus (P(t)); total organic carbon (TOC) was computed. The results showed that pH levels in S1 and S2 remained below that in the control for most of the thermophilic stage. This may be related to water-soluble ammonia and the affinity of zeolites to ammonium ions. A significant decrease in the level of ammonia nitrogen in water extracts from S1 and S2 was observed between days 5 and 35 in comparison with the control. The values of ash also differed and corresponded to the intensity of the decomposition processes in the respective substrates.