In a series of perifusion (superfusion) experiments, data were obtained which indicated that LRH and K+, when effective, induced increase in LH and/or FSH release within about one minute after arriving in the vicinity of the test rat pituitary tissues. Hormone release stimulating effects of LRH or high K+ continued for some time after washout of test agent. To secure these data perifusion flow was set at 2.5 or more ml/min., each test solution was pulsed for two minutes only, effluent samples were collected at 15 second intervals shortly before, during and after each pulse, and both mixing and dead space time were minimized by introducing each new test solution directly in front of the tissue chamber, via a 4-way or 5-way pulse control stopcock through which two streams of perifusion fluid (Control and Test) passed continually. LH and FSH in effluent samples were estimated by radioimmunoassay.