['This study investigated the variation in plasma insulin concentration (PIC) in a group of Icelandic horses in training, considered to be healthy and examined possible relationships between PIC and gender, age, body size, body condition score and management factors such as feed allowance and subjective level of fitness.', 'Plasma insulin concentration ranged from 0.2 to 13.9\xa0mU/l, body condition score from 2.3 to 4.0 and concentrate allowance from 0 to 4\xa0kg. There was a significant effect of concentrate allowance (P\xa0=\xa00.0007) and body condition score (P\xa0=\xa00.004) on PIC. For every 1\xa0kg increase in the concentrate allowance, log-PIC increased by 0.26\xa0mU/l. For every 1 unit increase in body condition score, log-PIC increased by 0.45\xa0mU/l. There was no effect of hay allowance, level of fitness, transport time, body size and age on insulin concentration.'] Does plasma insulin concentration increase linearly with body condition in Icelandic horses?