The effects of various i.v. naloxone doses (10 microgram, 300 microgram, 600 microgram, 1 mg/kg) were studied upon vocalization threshold to foot-pressure in 30 normal rats and 70 rats with Freund's adjuvant-induced arthritis. In arthritic rats, the lowest naloxone doses yielded a clear increase in pressure required to trigger vocalization (i.e. were analgesic) while the highest dose (1 mg/kg) was hyperalgesic. In normal rats, naloxone never induced significant changes in the vocalization threshold.