Sixteen native ram lambs weaned at 10 wk of age were divided into two groups. Eight animals were immunized against LHRH with a mixture of two fusion proteins: ovalbumin-LHRH-7 and thioredoxin-LHRH-7. The immunized lambs received a primary immunization plus two booster immunizations at 4 and 12 wks. Animals in the control group (n = 8) were not treated. Scrotal measurements and blood samples were taken at 2-week intervals. Beginning at 25 wk of age, semen was collected and sexual behaviour was evaluated on a weekly basis. At 35 and 37 wk of age testes and accessory glands of all animals were subjected to ultrasound scanning. At 37 wk of age animals were slaughtered and testes were evaluated histologically. Serum LHRH antibodies (P < 0.01) were detected in animals of the immunized group which had reduced serum testosterone concentrations (P < 0.01). Testicular development was suppressed in the immunized animals (P < 0.01). Immunized animals exhibited mounting activity 5 wks later than control animals. No mature spermatozoa containing ejaculates were collected from immunized animals. Control animals had moderately echogenic ultrasonographic appearance at 37 wk age, whereas immunized animals had hypoechogenic images. Mean seminiferous tubule diameter in immunized lambs was significantly smaller than that in control lambs. Basal membrane was thickened and hyalinized; there was an increase in peritubular connective tissue. No proliferating spermatogonia or mature spermatozoa were present in the tubules in these animals. There were no differences in the ultrasonographic appearance of prostate and vesicular gland between control and immunized animals. The LHRH recombinant fusion proteins were effective in immunological castration in ram lambs when started at 10 wk of age as noted by differences in serum testosterone, testicular histology and ultrasonographic appearance of testis and weight of accessory sex glands. Determining the effects of immunization on ultrasonographic appearance of the testis related to time after immunization requires further investigations. (c) 2004 Elsevier B.V. All rights reserved.