Several peptides found in hydrolysed poultry byproduct meal can inhibit angiotensin-converting enzyme (ACE) activity, a property that indicates potential antihypertensive and health-promoting effects. This study aimed to assess the effects of extruded diets containing enzymatically hydrolysed poultry byproduct meal (HPM) on cat serum ACE activity and aldosterone (ALD) concentration, nutrient digestibility, and faecal characteristics. On the basis of a preliminary in vitro ACE inhibitory activity assay, a commercial HPM and a commercial conventional poultry byproduct meal (CPM) were selected for further investigation. Two isoenergetic and isonitrogenous diets were formulated: CPM diet (25.7% CPM) and HPM diet (24.7% HPM). In trial 1, the effect of diet on serum ACE activity and ALD concentration was evaluated using 8 healthy cats (4 female and 4 male, 4.1 ± 0.38 kg BW) in a crossover design, with 5 d of adaptation and blood collection on d 6. In trial 2, apparent total tract digestibility and faecal characteristics were evaluated using 12 cats (6 female and 6 male, 4.0 ± 0.72 kg BW) in a completely randomised design. Serum ACE and ALD were analysed using a mixed model, with diet as the fixed effect and cat as the random effect. Data from trial 2 were subjected to analysis of variance, and means were compared by Tukey’s test. In vitro ACE inhibitory activity of HPM (90.4%) was higher than that of CPM (52.0%). Cats fed the HPM diet tended to have lower serum ACE activity than those fed the CPM diet (126 versus 142 U/l, p = 0.09). Serum ALD was not influenced by diet. Diets had similar digestibility values, and faecal consistency scores tended to be higher (firmer faeces) in cats fed the CPM diet than in cats fed the HPM diet (4.6 versus 4.0, p = 0.09). Inclusion of HPM in extruded diets may reduce cat serum ACE activity and promote good faecal consistency without affecting digestibility. Further investigations are needed to explore the potential health benefits of HPM in hypertensive cats.