Precise application of physiological stimuli and accurate measurement of physiological effects might require invasive procedures. If several functionally equivalent procedures are available, a decision should be made as to which is least likely to have a negative effect on psychological well-being. For example, blood can be taken from indwelling catheters or by venipuncture. When samples need to be obtained with high frequency and minimal disturbance, a catheter or vascular access port might be preferred. Taking blood via catheter or vascular access port causes no pain, but the animal should be anesthetized while the equipment is being installed and should be prevented later from pulling it out. A primate chair can prevent the animal from removing the catheter, but it severely restricts the animal's mobility. Another method that might protect the catheter but cause less restriction of movement is a tether system that includes a protective cover on the catheter.
The stress involved in venipuncture lies primarily in the physical restraint necessary to obtain the sample. A variety of nonhuman primates have been successfully trained to extend a limb voluntarily to permit a sample to be collected if repeated brief sampling is required (Bernstein and others 1991; Laule and others 1996; Rose and others 1975). For some purposes, hormonal data can be obtained from samples of urine and feces, rather than blood samples (Crockett and others 1993b; Kelley and Bramblett 1981; Lopez-Anaya and others 1990; Lunn 1989). Baboons have also been trained to submit routinely for taking blood pressure (Turkkan and others 1989). In addition, a variety of devices are available, including commercial products, for telemetric recording of some physiological characteristics and for the delivery of stimuli or active substances. Personnel safety is paramount when people are working with and training nonhuman primates, and no single technique for gaining access to an animal's arm or leg safely is universally accepted. What is considered the best safe practice is to train the animal to extend its limb (and sometimes to hold a firmly attached bar with the fist), or place a limb or shoulder against the cage, but persons should NEVER reach into the cage of any but the smallest species. Macaques, chimpanzees, and other large species can do great damage to those who fail to heed this tenet. Readers should become very familiar with the training literature before initiating these programs (see ''Restraint and Training" in Chapter 3 and Laule and others 1992).
Investigators should always be mindful of the effects of procedures on the well-being of the animals involved. If a choice is possible, instrumentation that appears least likely to cause a subject discomfort or distress should be chosen.