Nitric oxide (NO) is a signalling molecule that contributes to the control of many physiological pathways, including the heat-loss response of skin vasodilatation. Recently, NO has been implicated in the control of sweating during exercise in young adults. We tested the hypothesis that ageing reduces NO-dependent sweating during exercise in the heat. Ten young (23 ± 3 years old) and 10 older men (64 ± 5 years old), matched for body surface area, performed three successive 15 min bouts of exercise (Ex1, Ex2 and Ex3) at the same rate of metabolic heat production (300 W m−2) in the heat (35°C, 20% relative humidity). Exercise periods were interspersed with 15 min recovery periods. Local sweat rate (ventilated capsule) was measured on two forearm skin sites, which were continuously perfused via intradermal microdialysis with 0.9% saline as control (CON) or 10 mm NG-nitro-l-arginine methyl ester (l-NAME), a non-selective NO synthase inhibitor. Local sweat rate at the end of Ex1 was lower in the CON conditions in the older versus young men (0.69 ± 0.19 versus 0.90 ± 0.17 mg min−1 cm−2, P = 0.018). In the young men, local sweat rate was reduced in the l-NAME-treated conditions compared with the CON conditions at the end of Ex1 (0.67 ± 0.14 versus 0.90 ± 0.17 mg min−1 cm−2, P = 0.004), Ex2 (0.78 ± 0.20 versus 1.03 ± 0.20 mg min−1 cm−2, P = 0.013) and Ex3 (0.78 ± 0.20 versus 1.03 ± 0.21 mg min−1 cm−2, P = 0.014). In the older men, there was no main effect of treatment conditions on local sweat rate (P = 0.537) such that local sweat rates in the l-NAME-treated and CON conditions were similar (Ex1, 0.65 ± 0.20 versus 0.69 ± 0.19 mg min−1 cm−2; Ex2, 0.80 ± 0.27 versus 0.91 ± 0.29 mg min−1 cm−2; and Ex3, 0.84 ± 0.31 versus 0.94 ± 0.38 mg min−1 cm−2). We conclude that ageing attenuates the influence of NO in the control of local forearm sweating observed in young adults during short 15 min bouts of exercise in the heat. This mechanism may, in part, explain the age-related impairments in sweating.