Growth and thyroid hormone levels were recorded following chronic passive immunization against somatostatin (SRIF) in lambs reared under warm or cold environmental conditions. Forty 4-d-old Suffolk lambs (20 male, 20 female), with ewes, were reared outdoors under cold (CLD; −12.1° ± 1.6 °C) conditions or indoors under warm (WRM; 16.7° ± 0.2 °C) conditions. At regular intervals, beginning at birth, lambs in each group were administered (s.c.) ammonium sulphate-purified ovine immunoglobulin raised against ovalbumin (control) or a SRIF-ovalbumin conjugate (anti-SRIF). The anti-SRIF immunoglobulin demonstrated a dissociation constant of 15.9 pM. Lamb weight was recorded and blood samples were harvested at regular intervals from 4 to 25 d of age. Cumulative gain was reduced in CLD lambs at 7 (P < 0.001) and 10 (P < 0.01) d of age. Serum triiodothyronine (T3) was higher (P < 0.02) in CLD lambs from 4 to 20 d of age. SRIF immunoneutralization (25% binding of labelled SRIF at a 1:5000 serum dilution) did not affect lamb growth. Serum T3 was transiently elevated (P < 0.09) in anti-SRIF lambs after exposure to cold. No consistent influence of SRIF immunoneutralization or a cold environment on serum thyroxine (T4), cortisol or insulin-like growth factor I was observed. SRIF immunoneutralization did not enhance growth rate of the rapidly growing lamb reared under warm or cold environmental conditions. Key words: Sheep, somatostatin, immunoneutralization, cold, thyroid, IGF-I