Four experiments examined blocking and UR diminution in human SCR conditioning. In Experiment 1, CSX was conditioned in compound with previously conditioned CSA, in one group, or in a compound with another novel stimulus, CSN, in additional group. UR diminution occurred, but there was no post-compound difference in CRs in the two groups and no difference in responding to CSX and CSA following equal training on each, i.e., no blocking. The correlation between blocking and UR diminution was not significant. In Experiment 2, two groups received a blocking procedure, but one was tested on CSX and one on CSA. UR diminution occurred, but there were no post-compound CR or UR differences. There was again no within-subjects blocking effect. The correlation between blocking and UR diminution was not significant, but this correlation was significant when data from Experiments 1 and 2 were combined. In Experiment 3, a differential conditioning procedure was used, followed by reinforcement of two compounds, one with CSA+ and one with CSA-. One subgroup was tested on CSA+ and CSA- with reinforcement; another was tested without reinforcement. UR diminution occurred, and there was significant blocking, but only after two nonreinforced tests. Blocking and UR diminution were significantly correlated. In Experiment 4, four groups received compound conditioning after initial training with CSA. The compound contained CSX and CSA for three groups and two novel CSs for the fourth. Ten nonreinforced test trials were run. The three blocking groups received two CSAs plus eight CSXs four CSAs plus six CSXs, or all CSXs. The fourth group received ten nonblocked CSXs. CR magnitude to blocked CSX was significantly smaller than to nonblocked CSX during the last six test trials, but not initially. Blocking and UR diminution were not correlated, in disagreement with the first three studies. The results indicated that repeated nonreinforced test trails are needed for blocking to be observed in this preparation. The proposition that blocking and UR diminution are both due to reduced processing of the US received only partial support.