Blood samples and testes were collected from long-finned pilot whales (Globicephala melas) off the Faroe Islands at irregular intervals over a period of 3 years (July 1986-December 1989). Changes in testis mass (n = 674) and plasma testosterone concentrations (n = 214), measured by radioimmunoassay, were examined with respect to age, bodylength and bodymass of the animals. Corresponding to a rapid testicular growth (from 0.25 kg up to 1.9 kg), puberty occurred in male pilot whales of 4.6-5.7 m in bodylength, 1.2-1.9 tonnes in bodymass and 11-22 years of age. Changes in plasma testosterone concentrations confirmed this result, with very low values (< 2 ng ml-1) in immature animals (testis mass < 0.2 kg), followed by a sharp increase (from 2 to 29 ng ml-1) during the pubertal period, and the maintenance of high concentrations with large variability (> 1.5 ng ml-1 to 14 ng ml-1) in mature males. Testosterone concentrations were significantly correlated with testis mass (P < 0.001), but not with bodylength or age, and very large individual variations were observed in mature males. The average age, length and mass at the attainment of sexual maturity were estimated at 16.99 +/- 0.30 years, 5.162 +/- 0.013 m and 1.403 +/- 0.005 tonnes, respectively.