Neurons exhibiting reduced nicotinamide adenine dinucleotide phosphate-diaphorase activity (NADPHd) were quantified at 500 microns rostrocaudal intervals in spinal trigeminal nucleus (Vsp) of adenalectomized (ADX), ADX + corticosterone, and sham-ADX rats 6-12 days after surgery. NADPHd neurons were found predominantly in Vsp subnucleus caudalis (Vc) and in dorsomedial subnucleus oralis. ADX significantly increased the number of NADPHd neurons in superficial laminae of Vc, an effect reversed by chronic corticosterone replacement. ADX effects on NADPHd in superficial laminae of Vc but not in deep laminae of Vc or in the periobex region of Vsp paralleled previously observed sites of ADX enhancement of noxious stimulus-induced Fos-like immunoreactivity. The results indicate that chronic changes in adrenal steroid status regulate NADPHd, a mechanism that may both derive from changes in nitric oxide synthase expression and influence the processing of nociceptive information by central trigeminal neurons.