The effect of heat on the availability of lysine in field peas (Pisum sativum cultivar Dundale) was determined using the slope–ratio assay with growing pigs. The field peas were heated to 110°, 135°, 150°, or 165° for 15 min using a forced-air dehydrator. Lysine availability was significantly depressed (P < 0.05) with the application of heat, even at mild temperatures of 110°. Lysine availability values of 0.96, 0.71, 0.77, 0.56, and 0.47 were determined for the raw peas and peas heated to 110°, 135°, 150° or 165° respectively. The effect of dietary protein level on the ileal digestibility of lysine in raw field peas was determined in a second experiment to estimate true ileal digestibility. Five diets were formulated to contain 85, 105, 125, 145 and 165 g protein/kg. Increasing dietary protein from 85 to 105 g/kg resulted in a significant increase in the estimate of apparent ileal digestibility from 0.84 to 0.93. Comparisons were then able to be drawn between the ileal digestibility, availability and utilization of lysine from raw and heat-treated field peas. The results indicate that unlike ileal digestibility values, estimates of lysine availability are sensitive to heat treatment and are a close reflection of lysine utilization in heated protein concentrates. Thus, lysine availability values would be more suitable for use in diet formulations than ileal digestibility estimates when dealing with heat-processed protein concentrates.