The purpose of this double-blinded study was to examine the effects of a 1% hydrocortisone, leave-on conditioner on haematological and biochemical parameters, adrenal function tests, and cutaneous reaction to serial dilutions of histamine phosphate in normal dogs and those with pruritic dermatitis. Groups 1 and 2 each consisted of eight normal dogs. Seven pruritic dogs comprised Group 3. All dogs were bathed twice weekly for 6 weeks. Groups 1 and 3 had 1% hydrocortisone conditioner applied after each bath. Group 2 had vehicle from the conditioner applied after each bath. The amount of 1% hydrocortisone applied to the treated dogs ranged from 278 to 416 mg/m2. Haematological and biochemical analysis and adrenocorticotrophic hormone (ACTH) stimulation tests were performed on all dogs on days 0, 14, 28, and 42. Mean values for all blood and serum parameters remained within normal limits during the study. Post-ACTH cortisol levels were significantly lower in Group 3 compared to Groups 1 and 2 on day 42 (P < 0.05) and when averaged over all days of the study (P < 0.05 and P < 0.01, respectively). Serum alkaline phosphatase levels were significantly lower in Group 3 on day 0, compared to days 14 (P < 0.05), 28 (P < 0.01), and 42 (P = 0.05). All dogs received intradermal injections of buffered saline and five serial dilutions of histamine phosphate on days 0, 14, 28, and 42. No significant differences were apparent among the groups in subjective and objective evaluation of intradermally injected dilutions of histamine. In this study, the use of a 1% hydrocortisone leave-on conditioner did not result in clinically evident adverse effects, and only minor changes in blood parameters were detected. Although mean values in all groups remained within reference ranges throughout the study, the finding of statistically significant lower post-ACTH cortisol concentrations in the pruritic dogs (Group 3) suggests that absorption of hydrocortisone may have occurred. The results of this study also show that this product does not significantly suppress cutaneous reactivity to histamine in normal and pruritic dogs.