Lamb rennet pastes were prepared by the procedure most commonly used by Idiazabal cheese manufacturers. We studied the effects on their coagulating and lipolytic activities of the state of the stomach at the time of death (full of milk or empty), the amount of NaCl added, the origin of the lambs and paste storage time. Coagulating activities were generally between 155 and 363 units/g tissue. Pastes prepared from stomachs of lambs from slaughterhouse flocks had significantly higher coagulating activities than those of lambs from separate flocks. No significant decrease in coagulating activity was observed after 1 year storage at 4 degrees C. Chymosin represented 75-80% of the total coagulating activity with the remainder being pepsin. Rennet paste extracts with pH < 4.7 did not have increased coagulating activities when their pH was lowered to 2.0, while those with pH > 5.2 had activities 1.5-fold those before treatment. Lipase activity was higher in extracts of rennet pastes prepared using the stomachs of lambs that arrived at the slaughterhouse in the morning just prior to slaughter than in those prepared with the stomachs of lambs that had arrived on the previous evening. However, the reverse was the case for esterase activity. Activating the coagulating activity by pH cycling completely destroyed both lipolytic activities. Storage at 4 degrees C for > 1 year did not affect esterase activity but lipase activity decreased substantially after 4-5 months. Lipase, but not esterase, activity was responsible for the liberation of short-chain free fatty acids from ovine milk fat.