1. Growth hormone-deficient hypopituitary adults often complain of weakness and fatigue. The cause of the fatigue is unknown but could be an increased proportion of fast, fatiguable, type 2 fibres in the muscle. The aim of this study was to examine the contractile properties of the quadriceps muscle in a group of these patients compared with healthy controls. Changes in these properties were also examined in a small subset of the patients following growth hormone replacement. 2. Isometric strength, half-relaxation time from a twitch (t1/2) and the force-frequency relationship were measured using electrically evoked contractions in 14 growth hormone-deficient patients and 14 age- and sex-matched controls. Six patients were restudied following 6-24 month's replacement therapy with growth hormone (daily dose 0.04 +/- 0.01 i.u./kg). 3. The growth hormone-deficient patients had a significantly lower t1/2 than the controls (46.1 +/- 6.1 ms versus 56.1 +/- 10.5 ms respectively; P = 0.0072; mean +/- SD). The 10/100% ratio was also significantly lower in growth-hormone-deficient patients (38.6 +/- 9.9% versus 52.3 +/- 8.0%; P = 0.0005), as was muscle strength (349 +/- 99 N versus 493 +/- 215 N; P = 0.036). Following growth hormone replacement, muscle strength increased significantly (P < 0.05). The 10/100% ratio also increased towards control values, but this change was not significant. 4. These results demonstrate that the relaxation times of the quadriceps are significantly shorter and that the force-frequency relationship shifted to the right in growth hormone-deficient patients, which is consistent with a greater proportion of type 2 fibres within the muscles.(ABSTRACT TRUNCATED AT 250 WORDS)