The actions of hypothyroidism on BS, serum IRI and circulating FFA profiles observed in response to single glucose pulses at three levels of stimulation (1.00, 0.66 and 0.33 g/kg body weight) in male dogs were studied. Hypothyroidism modified neither of the mean basal values of these variables. There were different mean BS responses for every time and dose with significant interaction between the two. The BS curves at different doses were not parallel. There was a different time effect for every dose of glucose, and normal and hypothyroid dogs did not differ in this respect. The mean serum IRI responses found in normal and hypothyroid dogs were different; the mean responses at different times also differed, and there were significant normality-time and dose-time interactions. If the hypothyroid dogs and the euthyroid controls received a particular dose of glucose, a significant time effect on the serum IRI level was observed. In the normal dogs receiving glucose dose 1.00, a significant serum IRI response between 5 and 25 min was observed; in the hypothyroid dogs receiving a similar treatment, the significant response lasted from 5 to 45 min. In the normal dogs receiving glucose dose 0.66, the response was significant only at 5 min, and the serum IRI levels were below baseline between 60 and 90 min, while in the hypothyroid dogs the IRI response lasted from 5 to 25 min. In both normal and hypothyroid dogs receiving the 0.33 dose, the responses were significant between 5 and 25 min. As for the mean serum FFA responses to glucose, they were different at every time, and a significant normality-time interaction was found. In the euthyroid controls, the response lasted from 5 to 60 min, while it was longer in the hypothyroid dogs, lasting from 5 min after glucose injection until the end of the test.